The signaltonoise ratio (SNR) of space coherent optical communication system is greatly influenced by the laser linewidth. This paper compares and analyzes the influence of laser linewidth on the SNR of the heterodyne detection output when the time difference τd that two beams arrive at the photosensitive surface of the balanced detector and the detector characteristic parameter are different. Theoretical analysis and experiment result show that the differential frequency signal bandwidth increasing caused by laser linewidth broadening is independent withτd. When the differential frequency signal bandwidth increases by 2.673 times, the output SNR decreases by 0.369 2 times, which is approximately equal to 1/2.673. Increasing the internal gain, response degree and response bandwidth of the detector can slow down the SNR decreasing speed as the laser linewidth increases; however, when the internal gain and response bandwidth are too large, corresponding noise signal is also stronger, which is not benefit to the improvement of the SNR.