A selection algorithm of mode selection threshold is proposed to applied to adaptive modulation coding technology for atmospheric laser communication is proposed. The performance of atmospheric weak turbulence channel is analyzed, the threshold selection algorithm for Gauss channel is modified. A selection method named signal to noise ratio-turbulence intensity is proposed to determine the mode for threshold region, and a equation for threshold selection is presented. The bit error rates of the adaptive modulation coding system are analyzed under three transmission modes when the turbulence intensity is 0, 0.1, 0.2, 0.3 respectively. The corresponding relationships between signal-to-noise ratio and turbulence intensity threshold region of the three transmission modes are obtained under the condition that the maximum bit error rate is 10-4. Simulation results show that the proposed method is feasible, and it can accurately realize mode selection.