A detectable intermediate frequency current signal is generated by mixing the signal beam with the local oscillator beam in a coherent detection system. The signal beam which carries the target information is amplified and frequency shifted to allow operation at a sensitivity that approaches the shot-noise limit. To make full use of the return signal and achieve preferable performance，the mixing characteristics should be considered. Signal and local oscillator beam incident on the surface of photo-detector, and the influence of fields distribution on mixing efficiency was discussed in this paper. The highest intermediate frequency current could only be achieved when the field distribution of the signal beam was completely identical with that of the local oscillator beam. For both the signal and local oscillator were Gaussian beam coherent in different conditions, such as focus offset, beam waist differecnce and beam waist mismatch, the mixing efficiency was theoretically analyzed and numerically. Numerical simulation shows that the mixing efficiency is more sensitive to the alignment of the incident signal beam than that of the local oscillator beam, the radius of the local oscillator beam waist should be equal or slightly larger than that of the signal beam, the detector radius should be double that of the local oscillator beam waist.