Using the phase operator, the expression of the phase of Bose-Einstein condensate is obtained, which associates with time, initial phase, non-interaction energy and the atom number. Further, the interference condition of two condensates is given after calculating the relative phase of two condensates with interaction. Finally, it is found that the phase of condensate is mainly determined by the potential well for a finite atom number with the TF approximation, which is similar to the cavity of optical laser.