The coupling effect of quantum noise terms in a nonlinear laser field is analysedtheoretically with a two- dimensional gas laser model. The steady- state distribution function of the laser intensity and phase is derived through a Fokker-Planck equation. The mean, variance and skewness of the steady-state laser intensity and phase are calculated. Compared with the laser model of independent noise terms, the coupling of quantum noise terms changes the fluctuations of either intensity or phase drastically, it also introduces coupling fluctuation between the laser intensity and phase.