Adopting linear approximation method, the normalized steady-state mean intensity fluctuation of a single-model laser driven by pump noise and quantum noise with cross-correlated real and imaginary part was calculated, and the valid range for the linear approximation method was analyzed in detail. The influences on the normalized steady-state mean intensity fluctuation by the cross-correlation coefficient between the real part and imaginary part of the quantum noise, the intensities of the pump and the quantum noise, the amplitude and frequency of the input signal, and the net gain of the laser were discussed. It was found that the precision for the linear approximation is high in the case of weak cross-correlation between real and imaginary parts of the quantum noise, small noises, far from the threshold, small amplitude and high frequency of the signal.