Because of the coherent superposition of the diffraction light,wave-front on different region of the mask pattern has different impact on field point in the resist. In order to analyse the contribution of these regions,a theoretical model based on wave-front division was presented. The mask pattern was divided into sub-regions and the impact of their wave-fronts on the diffraction field was investigated. For interference cancellation of the diffraction light,a special region can be finally attained,in which the wave-front has the most contribution to the field point. Study results show that,for the internal point of the pattern,the size of the region is a half-wave zone around the field point; and for points near the edge,the region is generally in a size larger than a half-wave zone,and its scope depending on the shape of the pattern. It is of great significance to improve the simulation efficiency of resist shape error caused by diffraction in lithography. Experiments have verified the calculation results.