A non-contact liquid viscosity sensor based on microfluidic fiber-optic tweezers is proposed, which can realize precise viscosity measurement based on the stable trapping and dynamic analysis of single cells by using a tapered fiber probe. The optical potential well is formed by the fiber tip to capture the target cell, and the backscattered light signal is detected to monitor the dynamic response of the cell under controlled fluid flow. By analyzing the equilibrium between the optical restoring force and the viscous drag force, the variation time of the signal is extracted to quantify the liquid viscosity. Experimental results demonstrate a sensitivity of 4.49 ms/(mPa·s) and a linearity better than 98%. This method provides a novel, non-invasive platform for precise viscosity sensing in biomedical and clinical diagnostics.