The surface shape data of optical interferometry are required when the optical components are iteratively machined using equipment based on Computer Controlled Optical Surfacing(CCOS).However,when the off axis concave parabolic mirror is measured using the autocollimation detection optical path without image handicap method,the projection distortion caused by the optical path distorts the detected surface map,which makes it difficult to quickly converge on the surface quality of the workpiece.To address this problem,a distortion correction method for off axis concave paraboloid mirror based on autocollimation detection optical path is proposed in this paper.By determining the projection coordinate transformation of CCD coordinate system and mirror coordinate system in the interferometer,and combining with the calibration of a few mark points,the whole surface shape is reconstructed to correct the projection distortion.According to the autocollimation detection of Φ 430mm off axis concave parabolic main mirror of a large diameter off axis quad mirror optical system,the surface shape accuracy RMS reaches λ/80(λ=632.8 nm)by using the detection data after distortion correction as the machining guide of Mr Equipment.This method achieves high precision surface shape correction based on a small number of marks,which can effectively shorten the distortion correction time in CCOS iterative machining,and improve the machining efficiency of optical components while ensuring the surface quality requirements.