Microfluidic technology has developed rapidly in the past few decades and has been widely used in chemical synthesis, drug delivery, bioanalytical and optical technology applications. Accurate flow rate detection, as one of the key technologies, is used in several fields, such as controlling the efficiency of cell counting and sorting in flow cytometry, influencing the immunoreaction between antibodies and targets in the immune system, and enhancing the precision of many chemical or biological sensors. Optical sensors, which have the advantages of high sensitivity, simple fabrication, resistance to electromagnetic interference, chemical resistance, and short response time, have received great attention in flow rate detection applications. Particularly, Whispering Gallery Mode (WGM) optical microcavity has become an ideal platform for highly sensitive flow rate sensing owing to its high quality factor (Q) and small mode volume that can confine the photons to circulate millions of times within the cavity to effectively enhance the light-matter interactions, thereby significantly improving the detection sensitivity and resolution. WGM microcavity flow rate sensor is primarily based on two principles, one is based on the thermal effect of fluid flow, relying on the resonance wavelength shift induced by temperature variations to detect flow rate, and the other is based on the Bernoulli effect principle of ideal fluid, utilizing pressure sensor mechanism for flow rate detection. The former usually requires the use of a high-power light source or modification of the microcavity structure to increase the initial temperature, while the latter obviates the need for high-temperature conditions, thereby reducing experimental equipment costs and simplifying operation. However, in practical microfluidic applications, the size of the microfluidic channels and devices are usually in the micro-nanometer scale. Due to the internal viscous loss of the fluid, the energy loss caused by the viscous resistance during fluid flow is non-negligible. In this paper, a flow rate sensor based on WGM microbubble cavity is proposed and experimentally validated employing Bernoulli effect principle for viscous fluids.The WGM microbubble cavity flow rate sensor uses fiber taper for coupling to excite the resonance modes, with the coupling system comprising vertically aligned the microbubble cavity and the fiber taper. In order to better explain the sensing principle, a two-dimensional rotational symmetry model is established using the finite element simulation software, and the optical field distribution of fundamental modes, radial second-order modes and third-order modes under different wall thicknesses are simulated. The simulation results show that there is a light field distribution within the inner wall region, so that the light field can interact with the material inside the microbubble cavity. Furthermore, reducing the wall thickness of the microbubble cavity or adopting high order mode can enhance the sensitivity of the sensor. Subsequently, the pressure loss caused by viscous loss of the fluid is analyzed theoretically through Bernoulli effect equation. Among them, the pressure loss due to friction loss along the flow path exhibits a good linear dependence on the flow rate, while the pressure loss arising from local resistance demonstrates a quadratic relationship with the flow rate. The relationship between the pressure change and the flow rate change in different flow rate ranges is studied, which verifies the above theory and shows that the friction loss along the flow path is the main factor contributing to the pressure loss. In addition, the velocity field distribution and pressure field distribution of the microbubble cavity at the flow rate of 100 μL/min are simulated. It is observed that there is a maximum velocity at the center of the capillary, which is about 3 m/s, while the minimum velocity is at the boundary of the model. And the pressure field distribution shows that there is a highly uniform positive pressure distribution inside the microbubble cavity, leading to the redshift of the resonance mode. Finally, a microbubble cavity with a wall thickness of 2 μm is experimentally prepared, whose Q-factor can reach 10⁶ after filling with DI water, and the flow rate sensing test system is constructed. The pressure pump and flow rate sensor are employed to achieve accurate control of different flow rates, and the long-term stability at the flow rate of 10 μL/min is tested experimentally. The standard deviation of resonance wavelength shift within a period of 12 minutes is obtained as 0.029 pm, indicating the excellent stability of the test system. Furthermore, a good linear relationship between the resonant wavelength shift and flow rate of DI water is shown when varying different flow rates. The experimental results demonstrate that the flow rate sensitivity can reach 0.047 pm/(μL/min), with a detection limit of approximately 0.635 μL/min. The proposed flow rate sensor leverages the natural microfluidic channel of the microbubble resonator without any complicated modification of the device itself, which has the advantages of simple preparation, compact structure size, low cost as well as easy integration. And the viscous loss in practical microfluidic applications is taken into account, providing a new idea for realizing the detection of fluid property.