A new intensity modulated optic fiber acoustic sensor model has been proposed based on the assumption that the distribution of the light intensity from transmission fiber is Gaussian distribution. The model is based on single mode fiber collimator coupling characteristics theory, contains clamped circular flat MEMS membrane, and uses GRIN lens to realize the modulation of light intensity. The simulation results show that through choosing proper modulation parameters the sensitivity of the sensor can improve in orders of magnitude than that of the conventional intensity modulation model.