With the key technologies breakthrough of phase encoding, it is necessary to analyze the bit error rate performance of phase encoding optical code division multi-access accurately. A non-Gaussian approximation method, based on the moment generation function of the decision statistic and saddle-point approximation, is introduced to evaluate the statistic characteristics of multi-access interference, shot noise and thermal noise and cross-correlation. Mumerical comparison between Gaussian approximation method, accurate calculation and non-Gaussian approximation method, proves the high accuracy and efficiency of the proposed method to analyze the bit error rate of the phase encoding system.