Based on the enhanced effect of spontaneous emission in cavity quantum electrodynamics, the microcavity effect of vertical cavity surface emitting lasers (VCSELs) is analyzed, and the results obtained in ordinary open cavities and in three-dimensional closed cavities are compared. The rate equations for multi-quantum well VCSELs are deduced theoretically; and its output characteristics,i.e., the relationships of population inversion and optical power utputversuspumping rate, are discussed. At the same time, the ways to further reduce the simulated emitting threshold for semiconductor lasers are proposed.