Abstract: On the basis of Rytov variance, the log-intensity variance when partially coherent beam goes through strong turbulenc with Andrews model is deduced . The result shows along with the light source coherence becomes worse, that is the light becomes partially coherent light,the log-intensity variance on the receiving plane decreases. When the partially coherent light in multiple beams are used, the log-intensity variance is distinctly improved. But the transmission distance of communication is limited, when the coherence is bad. Therefor, the value of coherence parameter must be chosen properly.