Abstract: According to the definition of relative distortion, a distortion measurement with inverse imaging method was proposed. We first placed the target on the focus surface of the unit-under-test (UUT), put the entrance pupil position of the UUT at the rotation axis position of the precise turntable, and rotated the UUT from the forward maximum field of view (FOV) to the negative maximum FOV, then measured 14 FOV points and finally obtained the distortion value of each FOV. Distortion value of the whole FOV could not be measured unless the entrance pupil position of the UUT was put at the rotation axis position of precise turntable strictly. The accuracy of focal length measurement is 2 m, and the accuracy of relative distortion measurement is 0.5.