Abstract: Based on the expression of diffractive field distribution of the Gaussian beams restricted by a rectangular aperture and focalized by a lens, the relationship between the width of the rectangular aperture and the spot size of the restricted beam, as well as the relationship between the aperture width and the power in the bucket,was numerically calculated with MATLAB. It was illuminated that the width of the centre bright fringe decreased along with the increasing width of the rectangular aperture when the effect of the aperture on the beam couldn't be ignored, however, the width of centre bright fringe step changed with increase of the size of the aperture when the effect could be ignorable, the width of the half maximum intensity of the centre bright fringe decreased along with the accretion of the width of the aperture and then it tends to a constant. The piecewise function of the relationship between the full width at half maximum(FWHM) of the centre bright fringe and the rectangular aperture width was put forward as well. The conclusions are useful for the design of a laser facility.