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.