In coherent imaging ladar system, the round-trip time and the scan rate cause the deviation between the received signal
and the transmitter beam in spatial directionality. Therefore the return signal angle lags behind the receiving optical axis
angle. This lag-angle effect will cause energy loss and even the loss of return signals, which would finally affects the
image quality. In order to overcome the impact of lag-angle effect on image quality, the detector with rectangular active
area has been presented to replace the circular detector. In this paper, the numerical simulation of heterodyne efficiency
based on rectangular detector is analyzed. And the simulation results show that heterodyne efficiency quickly declines
with offset of return signal along optical axis when lag-angle effect exists. When the offset is below 0.5 and the
length-width-ratio is around 1.5, the heterodyne efficiency is above 50%.