The influences of signal to noise ratio (SNR) on the ranging property of laser radar with constant threshold to noise ratio (TNR) are studied comprehensively by experiments. The results indicate that when SNR is much higher than TNR, the distribution of laser radar′s ranging data obeys the Gaussian distribution. And with the decrease of the SNR, the distribution gradually deviates from the Gaussian distribution, and its rising edge becomes steeper and the falling edge becomes more slowly. While the SNR is approaching the TNR, the distribution returns to the Gaussian distribution, but the missing rate increases rapidly. Based on the experiments, the half of the confidence interval with confidence probability of 68.2% is also introduced to evaluate the range accuracy of laser radar. The corresponding method for calculating the range accuracy is presented and used to evaluate the range accuracy of laser radar under different SNRs.