In this paper we derive the Fokker-Planck equation of a closed lasing system by using ordering techniques of operators, reduce it to an equation including merely field variables, and then obtain the Q function of the laser field. Through analyzing the Q function, we discover that,at the operating point, the dynamic behavior of the dipole moment between the lasing levels decisively influences the quantum statistical properties of the laser field. Under certain conditions we obtain a clear picture about the influence. The conclusion can be applied to a variety of laser systems, including those without inversion.