Aiming at the unknown design parameter of rotary symmetry aspheric surface, discuss forming parameter fitting method, research on axial symmetry aspheric design parameter testing technology. Use contact probe filometer to measure data, acquire symmetry axis and end curvatures initial value by polynomial fitting, use least square method and restraint variable-dimension method to carry out curve fitting for measurement data, acquire aspheric surface design parameter, compare fitting parameter forming data with actual measurement data, and acquire PV value of error curve and related forming error. Fitting analysis results show that: the method has advantage of fast convergence speed and high stability, fitting parameter can restore design parameter of machining curve, meet ultra-precision machining requirements of aspheric mirror, and support aspheric mirror machining quality valuation and follow on compensation machining with theoretical basis.