We report a model to describe the characterization of pump noise transfer in an laser oscillator consist of nonlinear saturable absorber. At the first, we obtained a linear superposition relation for modulation transfer function of amplifier part. By using the nonlinear characteristics of saturable absorber, a nonlinear quadratic equation for MTF of laser oscillator obtained The theory then validate with experiment and good consistency observed. © 2011 Optical Society of America.