In OCDMA systems,the cross correlation of address codes is a crucial parameter which can directly affect the performance of the codes. Research of the distribution of the correlation becomes important work. Using strict algebraic operation and statistical analysis,a tiny modification on Kwong's distribution formula of PC is proposed. According to the same methods and means,a new distribution formula of EQC is also proposed. Based on this new distribution formula,the error probabilities corresponding to different combination of codes are compared. Finally it is concluded that using the address codes of EQC without user 0 can improve the performance effectively.