A longitudinal coupling factor is introduced into the rate equation of vertical cavity surface emitting lasers (VCSELs), taking the longitudinal coupling between photon and carrier into account. By using this modified equation, the relation between threshold current and number of quantun wells (QWs) is analyzed. The results show that the effect of longitudinal coupling between photon and carrier is not negligible. Furthermore, an expression for VCSEL′s threshold current Jth is given, which indicates that the bigger the longitudinal coupling factor is, the lower the threshold current is.