为避免传统激光外差探测中强本振信号带来的散粒噪声影响，着眼于光子级别的弱本振激光外差探测信噪比分析。首先，将脉冲体制弱本振激光外差探测信噪比表达式应用于分段连续弱本振激光外差探测中，获得中心频率为 221 kHz外差信号的解析，且信噪比的理论计算和实验值吻合。其次，采用数据段细分功率谱累积技术，在相同的信噪比情况下，所利用的数据量总数降低为传统功率谱累积方法的 1/10，提高了数据利用率，且在功率谱累积次数较少时，信噪改善比近似遵守 m规律。

In order to avoid the influence of shot noise due to the powerful local oscillator(LO) in traditional laser heterodyne detection, SNR (signal to noise ratio) analysis of laser heterodyne detection with a weak LO under photon-level is presented. Firstly, the expression of SNR with a pulsed laser is applied in the condition of partial CW(continuous wave) and weak LO laser heterodyne detection. The spectrum of the heterodyne signal with the center frequency 221 kHz has been achieved and the theoretical value of SNR is in agreement with that of the experiment. Secondly, the techniques of data segment fractionizing and PSD (Power Spectral Density) averaging are used in the research of SNR. The results showed that the volume of the data has been decreased to 0.1 times compared with the traditional PSD averaging, the data resource is fully used during signal processing and SNIR (Signal to Noise Improved Ratio) nearly obeys the m rule(m is the averaging time) under a small quantity of data segments.