A single_mode laser cubic model driven with quadratic pump noise is proposed. Closed laser intensity Langevin equation is got. Based on our early work of the Approximation Fokker_Planck equation (AFPE) for the systems driven by nonlinear external noise ［Cao L and Wu D J 1988 Phys. Lett. A 133 476］, the AFPE for laser intensity probability distribution is obtained. Exact analytic expression for the steady normalized intensity probability distribution is calculated. Based on the symmetry of steady normalized intensity probability distribution with respect to λ, the cross_correlation coefficient between the real and imaginary parts of pump noise, several general conclusions on the steady statistical property of laser system are reached.