In the monitoring process of human motion and posture, knee movement information was of great significance in the diagnosis and rehabilitation evaluation of chronic diseases in the middle and old age. The knee directional bending measurement device using a Mach-Zehnder interferometer-based directional bending (MZI-BDB) sensor was presented in this paper. The system consisted of MZI-BDB sensor, swept laser light source, optical fiber coupler, optical fiber isolator, photodetector and signal processing system. MZI-BDB sensor was fabricated by fusion-splicing a section of eccentric core fiber (ECF) between two single-mode fibers (SMF) with core-offset. It was encapsulated in a soft silicone sheet and fixed to the knee joint by a band. When the knee joint was flexed and extended, the MZI-BDB sensor was led to bend, causing the modal interferes of the sensor changes and the resonant wavelength shifted, so as to monitor the bending direction and curvature of the knee joint. The range of measurement of MZI-BDB sensor was 0°-90° at the positive and negative bendings respectively. At the positive bending (bending direction of 0°), the proposed MZI-BDB sensor sensitivity and resolution were 5.29 nm/m^-1 and 0.11 m^-1 respectively. The sensitivity and resolution were -3.11 nm/m^-1 and 0.12 m^-1 at the negative bending (bending direction of 180°). The temperature sensitivity of the proposed MZI-BDB was 0.043 nm/℃, which had minimal effect on the experiments. Furthermore, the photoelectric encoder and MZI-BDB sensor were used for data acquisition simultaneously. The experimental results show that the detection system and the photoelectric encoder verification platform are consistent in accuracy and responsiveness.