By adding periodic signal into single-mode laser, and taking the first-order approximation to the multiplication noise of the laser gain-noise model,we find that the output signal is of the Lorentz type when we obtain power spectrum by calculating the correlated function. The signal-to-noise ratio shows the stochastic resonance occurs with the variations of intensities of the pump noise and quantum noise, and the noise intensity of resonant peak is smaller than that under the zeroth approximation.