Taking partially coherent Gaussian- Schell model (GSM) beam as an example to get the best launch parameters of laser communication system, considering overall decline of the alignment adaption loss caused by beam drift and decrease of receiving optical power density induced by beam expanding, some numerical calculation and analysis on the receiving power in the laser link are necessary. Results represent that in atmospheric turbulence the best launch parameters of laser communication system are related to beam waist radius, spatial coherent length, wavelength, propagation distance and turbulence intensity. There exists an optimal value when the receiving power density of the partially coherent GSM beam changes with the variance of the wavelength, beam width, degree of coherence and transmission distance.