We investigate the performance of a free-space optical (FSO) coherent communication system in imprecise atmospheric composite channels, taking into account the combined effects of Malaga atmospheric turbulence, channel estimation errors, nonzero boresight pointing errors, and path loss. The probability density function (PDF) of the instantaneous signal-to-noise ratio of FSO systems using heterodyne detection technology in imprecise channels is derived. Based on this PDF, Meijer’s G functions are used to derive analytical expressions for the outage probability, average bit error rate, and average channel capacity of the quaternary phase-shift keying modulation system, to quantify the performance of the considered system. Through numerical simulation, the performance of FSO communication under imprecise and precise channels is compared and analyzed, and the effects of turbulence intensity, channel estimation errors, boresight error, spot jitter, beam divergence angle, and link distance on FSO coherent communication systems are evaluated. Finally, we validate the accuracy of both the simulation results and the analytical results through Monte Carlo methods.