Abstract: In the discrete numerical calculation of diffraction distribution, the total number of sampling points following sampling theorem increases with the augment of propagating distance. Although the traditional zero-padding method resolves this problem, the calculation load increases inevitably and PC's memory can not afford. A novel frequency shift interpolation, i.e., utilizing frequency spectrum calculation after shift in Fourier domain, was proposed. In the method, more sufficient spectrum components can be got without the increase of sampling number. A complete diffraction distribution is accomplished by splicing the light field obtained by spectrum of each shifted interpolation. The results show that the proposed method successfully evades the great sampling number caused by larger diffracting distance. Comparing with traditional zero-padding method, calculation load is decreased notably and the requirement to memory in numerical calculation is depressed.