The formula for calculating the signal-to-noise ratio is researched and showed in reference beam Laser Doppler Velocimeter. The effect of light spot size and size of light sensing surface on signal-to-noise ratio is analyzed from numerical simulation. Moreover, the formula of the Doppler current is given while the effect of the positional relation between reference beam and signal beam on Doppler current is analyzed. The theoretical analysis and simulation shows that the signal-to-noise ratio is an increasing function of light intensity of the reference beam, but when it reach a certain value, the signal-to-noise ratio increases slowly and is close to ηI_s/(hνΔf ). It can reach the maximum value 0.7ηP_s/(hνΔf ) when the Airy disk of two beams is nearly equal. The value of the Doppler current has a great correlation with the angle deviation of two beams when the two beams are non-parallel, whereas it has non-correlation with the incident angle of the beams when the two beams are parallel.