Based on Fresnel scalar diffraction theory, the analytical formula of far-field spot distribution of laser emission system in defocused state is derived, and a sampling interval optimization method based on similarity coefficient is proposed in this paper. The deviation between the analytical results from optimal sampling interval and numerical simulation is less than 0.04%. Fourier transform method is used to simulate the far-field spot distribution in the presence of aberration, and the influence of key parameters such as angular displacement, divergence angle, and gain on the light field distribution is analyzed. When the laser wavelength is λ, the allowable pointing deviation is 3 μrad, and the center gain margin is 3 dB, based on the tolerance allocation theory, the tolerances of defocus, tilt, spherical aberration, coma aberration, and astigmatism should be less than or equal to 0.36λ, 0.07λ, 0.43λ, 0.13λ, and 0.5λ, respectively.