A lower limb spasticity measurement method includes the step of setting the lower limbs of the person in a lower limb orthotic device of a gait activity machine, the step of starting up a motor of the gait activity machine to drive the lower limb orthotic device for lower limb activity, the step of getting a statistical distribution data from the output torque of the motor within a predetermined time and then calculating the statistical distribution data to obtain a threshold, and the step of determining whether the output torque of the motor is greater than the threshold or not, and then stopping motor if the output torque of the motor is greater than the threshold. Thus, the method of the invention can accurately measures spasticity in the lower limbs of a person without the use of sensors, effectively saving the cost of equipment.