We propose a calculation method of the bit error rate of space coherent optical communication link, to solve the influence of laser linewidth on the performance of space coherent optical communication system. The influence of the laser linewidth on the transmission error rate of the link in the turbulent atmosphere is studied, and the error rate model of the spatial coherent optical communication link in the atmospheric turbulence is established. The influence of different parameters on the bit error rate is analyzed, and the result of numerical settlement is given. It is found that, the linewidth increases by 1 times and the bit error rate increases by about 2.7 times; the linewidth increases by an order of magnitude and the corresponding bit error rate increases by nearly one order of magnitude.