Plane grating′s line error and surface error of substrate influence grating′s spectral performance, such as diffraction wavefronts, resolving power, ghost lines, satellites, scattered light etc. It is extremely important to study the relationship among grating performance, line error and substrate′s surface error. According to optical path difference caused by grating′s line error and surface error, math equations, which contain the relationship among line error, surface error, angle of incidence θ, diffraction order m and diffraction wavefront under condition of cone diffraction, are deduced and mathematical model of reconstructing the diffraction wavefronts of non-ideal grating is acquired. Based on above model, grating′s diffraction wavefronts of symmetric orders are measured by interferometer and the separation of grating line error and surface error of substrate are realized from measured results. Grating′s diffraction wavefronts are analyzed by two-dimensional fast Fourier transform method and the influence of grating line error and surface error on grating performance is discussed. With the aid of aforesaid method, grating′s resolving power, ghost lines, satellites, scattered light can be analyzed by reconstructing grating′s diffraction wavefront, and grating′s line error and surface error on the whole surface of grating can be deduced, both of which can provide a theoretical basis for grating′s substrate processing, grating manufacture and application.