Force/torque control of the hand exoskeleton has been proven to be more effective for stroke rehabilitation than its position control. However, the existing torque sensors used for small-scale exoskeletons still have limited performance. In this paper, a piece-wise stiffness model of the Archimedes spiral spring is first proposed, and the effects of the design parameters on the spring stiffness are analyzed. Second, the design parameter of the Archimedean spiral spring is optimized to improve the performance of the torque sensor. Third, a miniature torque sensor based on an elastic Archimedean spiral spring is proposed according to the index finger rehabilitation requirement. Compared with the existing state-of-the-art miniature torque sensors, the volume-to-torque ratio and weight of the proposed torque sensor can be reduced by 54.25\% and 57.7\%, respectively. Finally, comprehensive experiments are conducted to verify the proposed stiffness model and the performance of the proposed torque sensor.