Silicon photomultiplier (SiPM) is an array composed of hundreds or even thousands of single photon avalanche diode (SPAD). It has the advantages of high gain, easy integration into the array and anti-interference, and has a wide range of application prospects in LiDAR ranging. We analyzed the laser radar ranging signal and noise model, simulated the output of SiPM under sunlight, and obtained the corresponding mean value through Gaussian fitting. On the basis that the probability of being excited after SPAD received photons obeys the Poisson distribution, the calculation formula of SiPM analog output affected by background light when the background photoelectron is 0.001 /ns~0.01 /ns and the detector dead time is 5 ns~50 ns was given. Finally, we derived the expression of target detection probability. The experimental results of laser ranging detection probability under outdoor ambient light are consistent with theoretical calculations.