The invention relates to the technical field of robot performance testing, in particular to an industrial robot joint idle stroke testing device and a measuring method thereof. The industrial robot joint idle stroke testing device comprises a displacement sensor support and a displacement sensor, and the specific using steps of the device are as follows, S1, an industrial robot joint idle stroke testing platform is built; S2, the robot stops at the posture 1; S3, when the robot stops at the posture 1; S4, a torque is loaded; S5, the value of the displacement sensor is read; S6, the step 3 and the step 4 are repeated to obtain a joint angle of a posture 3; S7, the steps from the step 3 to the step 5 are repeated for three times, and three groups of alpha1 and alpha2 data are recorded; and S8, the joint idle stroke of a robot is calculated according to the recorded data. Compared with the prior art, the displacement sensor is used for multiple times of measurement to obtain multiple shaft joint angles, the measurement device is high in measurement precision, no extra measurement deviation is introduced except for connecting rod deformation of a robot body, meanwhile, operation is easy, convenient and rapid, and manpower, material resources and working hours consumed by robot disassembly and assembly are omitted.