Abstract: To achieve the measurement of bend stiffness of mechanical parts like harmonic reducer under high or low temperature,laser interferometer was used to measure the deflection angle of the output of harmonic reducer under the torque moment or bend moment which was forced on through wire ropes driven by motor.Results show that this design could solve the problem that angle coder cannot be applied under extreme temperature,and could eliminate the effect on the result of stiff measurement caused by system errors of self-deformation of instrument.The present measuring temperature is in the range between-65 ℃ and +80 ℃.