A method is proposed to calculate the focusing characteristics of short-wavelength laser beam through short-focus system. The problem of random phase jumps, wave function distortion and insufficient sampling rate will be met when the optical field distribution of a focused laser beam is calculated by fast Fourier transform method and short-wavelength laser beam passes through the short-focus system. By using coordinate transformation in the fast Fourier transform calculation, the limitation of the same input screen special scale, the same diffraction screen sptial scale and the same sampling grid spacing is broken. The optical-field distribution of a focused laser beam can be calculated in detail. At the same time, two-step calculation is used to avoid the conflict between the coordinate space of the geometric expansion and the diffraction limit when the light field near the focus is calculated.