Fiber optic tweezer is an effective method in biomedical applications that can be used to operate the objective in a microscale. It employs the moment changes when the photon passes through a small particle to generate a weak force on the particle. So the accurate control of the focus is the most important thing for the application of the tweezer. However, the tweezer is usually used in liquid environment. So the focus location measurement becomes extremely important and this cannot be easily obtained through the common CCD technology. In order to solve this problem, the Patch Clamp technology is employed, which has been successfully used in the electrophysiology field. In this technique, a micro pipette filled with the extracellular solution bearing a mega Ohm resistance, can give rise to a microampere current measuring accuracy. Based on the optical characteristics of the pure water and the FTIR result proves that the optical characteristics extracellular solution are quite close to pure water, 845 nm and 980 nm wavelength are chosen as the operating wavelength. The photothermal effect is generated by the solution absorbed the optical energy, and this effect can also cause the microampere current changes according to the Ohm′s law. In the experiment, the common communication fiber SMF28 is employed to fabricate a fiber-optic tweezer, and the light passes through the tweezer and incident to the solution. Following the scattering and absorption effect, the photothermal effect dominates the region of stimulation. When the micropipette filling with the solution is irradiated by the tweezer, the resistance of the micropipette will change determined by the temperature effect. Based on the scale of the micropipette, say 1 micro meter or so, the spot of the tweezer output can be measured through the micropipette movement under a precision control by a three-dimensional controller. In this way, a fine variable of the focus of the tweezer can be obtained when the wavelength switched.