Using the linear approximation method, the intensity correlation function C(t) of a single mode laser system was calculated and the time evolution of C(t) was researched as well as the influence of modulation signal frequency Ω and amplitude B on C(t) was analysed in detail. In the case of the pump noise selfcorrelation time τ<<1, it was found that when modulation signal frequency Ω and amplitude B increase, the time evolution of C(t) experiences monotonous descending; however, in the case of τ>>1, as modulation signal frequency Ω and amplitude B increasing, the time evolution of C(t) exhibits periodically surging with descending envelope.