Abstract: This paper analyses the diffraction characteristics of grating pair compressor in a CPA(chirped pulse amplification) system, using the 3D raytracing method, under the conditions that the grating grooves are unparallel while the grating surfaces are parallel. In this case, the group delay of optical light passing through this system, the second and the third dispersion of the system and the relative errors of dispersion with the condition that the grating grooves is strictly parallel were calculated. The reception position's variation quantity of the central wavelength light on the reception surface was achieved, meanwhile, its variations with the incident angle varying were analysed.