To describe the influence of the sensor microchannel structure on the output characteristics of the sensor, the relationship parameter of the sensor transverse and longitudinal microchannel length is introduced in this article. A mathematical model of galliumbased liquid metal flexible strain sensor is formulated by comprehensively considering the resistance change law of the liquid metal in the microchannels which are parallel and perpendicular to the strain direction in the sensor. According to the developed mathematical model, five strain sensor samples are optimized and prepared to perform the experiments within 40% strain range. The results show that the mathematical model developed in this article can well predict the output of sensors with different microchannel structures, and provide directions for the design and optimization of high sensitivity sensors. Experimental tests show that the designed strain sensor has a sensitivity of 2. 01, a hysteresis of 5. 1% at 40% strain, and a high reproducibility and output stability. The designed sensor is used for joint motion detection in the finger and elbow, exploring the application of gallium-based liquid metal flexible strain sensor in joint motion detection. This article provides theoretical support for the application of gallium-based liquid metal flexible strain sensors in the wearable field.