Abstract: A distortion measurement system, which is composed of high-precision goniometer, microscope measurement system, mini double-grating interferometer, collimator and star point, is used to measure the distortion of the NFOV long-focus lens precisely. Using third order polynomial fitting in center area of FOV, the theory focal length also can be computed. The measured deviation angle was corrected with the weighted mean of the high order of image height and the sine of the view angle at the edge field to acquire the relative and absolute distortion of each FOV. The results of the actual measurement and calculation show that the measurement accuracy of full FOV distortion can reach 0.02%.