With the key technologies breakthrough of phase encoding, it is necessary to analyze the bit error rate (BER) performance of coherent optical code division multi-access (OCDMA). The impacts of beat noise, multi-access interference (MAI), shot noise and thermal noise in coherent OCDMA are studied in this paper. A Gaussian approximation method, based on the moment generation function (MGF) of the noises, is introduced to evaluate the bit error rate by taking the polarization into account. Numerical result between beat noise and MAI is compared. When all the interference signals are parallel or upright with the targeted user, we derive the upper and lower bound of BER respectively, the BER of random polarization between them. It is shown that, the beat noise is the dominant factor which influence BER performance of the coherent OCDMA system. The impact of polarization is increasing with the receiving power increasing; when the BER is below 10-10, it increases quickly with the increasing of user number; when the BER is larger than 10-10, it changes slowly with the increasing user number.