Limited by view field angle, heterodyne system composed of single point detector cannot receive signal beams power adequately. The heterodyne system in which the single point detector is replaced by array detector can enhance the received amount of signal power through enlarging view field angle. Taking into account the thermal noise and saturation effects, signal-noise ratio(SNR) of heterodyne system with array detector is researched. Results show that compared with single point detection system, it will gain a higher SNR. However, it is pointed out, when the number of the units in array increases to a certain number, SNR cannot attain more improvement because of the thermal noise. For the reason that the signal output of system is the sum of output of all detector units, it is impossible to realize equiphase superposition. Impact of superposition in non- equiphase on system is also investigated by calculation simulation, and results show that the impact can be controlled effectively in the hardware design.