The presence of abnormal muscle retraction and contraction during the rehabilitation training poses significant challenges for patients with lower limb dysfunction caused by stroke, as it hinders their ability to regain lower limb function. Therefore, it is vital to detect spasm force during the training to adjust the force generated by rehabilitation robots. This article presents the design and development of a capacitive force/torque (F/T) sensor for lower limb rehabilitation to detect muscle spasm. The double differential measurement method and a decoupling structure with built-in crossed roller bearing were adopted to achieve high sensitivity and high accuracy. Calibration tests were conducted with a commercial torque and rotational speed sensor to verify the design and accuracy of the capacitive F/T sensor. Results show that the proposed sensor exhibits a linearity of 1.04%, a resolution of $0.05 {\mathrm{ N}}\cdot {\mathrm{ m}}$ , and a maximum measurement error of approximately 2.07%. Lastly, experiments were conducted to assess spasm detection capabilities in a lower limb rehabilitation robot that has capacitive F/T sensors installed on knee and hip joints.