A method of detecting reconstructed wavefront was introduced based on Hartmann principle sub-aperture slope scanning to solve image quality evaluation for large aperture optical system at different elevation angles. The method does not need the same size caliber standard mirror. The information of wavefront can be obtained by scanning. The optical software and mathematical analysis software were used to carry out computer simulation through DDE link. The simulation optical system used Cassegrain system with primary mirror 720 mm and secondary mirror 100 mm to verify the feasibility of this method. Random error inducing and multiple scanning averaging were used to study the accuracy of reconstructed wavefront. The error included spot centroid error, sub-aperture positioning error and sub-aperture tilt error. The comparison between the simulation results and the results of optical software was given, and the physical models of errors and reconstructed wavefront accuracy during error injection were obtained.