It is a key to analyze the electrical performances of antenna that the surface errors of distorted reflector are obtained accurately.An improved method to compute surface errors of antenna is presented against the problem caused by employing the unprecise method in ANSYS software to analyse the corresponding errors in distorted reflectors.The mathematic formulae to compute the errors in the axial,normal and radial directions of distorted surface are deduced in detail.The surface errors of a 7.3 m parabolic reflector antenna working under different conditions in engineering are anlayzed and calculated.The comparison of results by ANSYS software with that of improved method is obtained simultaneously.The simulation results verifiy the correctness and accuracy of improved method.Its application in engineering can reduce the cost of antenna deign and provide the precise error information of distorted antenna for electrical designer.