A mathematical model of the far-field beam distribution is built after the transmitted beam passes through the telescope with wave-front error. The effect of root mean square (RMS) value of wave-front error on the divergence angle of the transmitted beam is analyzed by numerical calculation combined with Monte Carlo method. Based on this, the tolerance of the telescope wave-front error required by the far-field divergence angle of the transmitted beam is determined. The analysis results show that for the laser communication system which the transmitted beam is off-axis on the telescope pupil, the far-field divergence angle is smaller than 10 μrad when the RMS of wave-front error is smaller than 0.13λ(λ is the light wavelength). The far-field divergence angle is smaller than 16.2 μrad when the RMS is smaller than 0.2λ and the probability of the divergence angle smaller than 10 μrad is 60%. The testing results of the wave-front errors of the telescope at different temperatures and the corresponding far-field divergence angles of the transmitted beams are well coincided with the analysis results.