To obtain the distortion of three-mirror off-axis,the calibration method and procedure of a system are presented,the principle and the composion of calibration system are also introduced.The distortion of three-mirror off-axi is analyzed,then a mathematical model for distortion calibration is established by means of least square regress method and the image distortion of a three-mirror off-axis is calibrated by combining a high accuracy turn table and a long focal length collimator.The results show that the calibration accuracy of absolute distortion is less than 2 μm(1σ),and the calibration accuracy of relative distortion is less than a 1×10^-4(1σ),which shows that proposed method can meet the request of high accuracy calibration of the distortion for three-mirror off-axis.It is valuable for the practical application.