Abstract: During gas reservoir developing, both reservoir effective pressure and gas saturation vary. It is significant for residual gas distribution predicting, using time-lapse seismic technology, to research on effects of effective pressure variation and gas saturation variation on seismic response characters. Acoustic properties between gas and oil are obviously different, and research results of oil reservoir time lapse seismic cannot be applied to gas reservoir, so it is necessary to develop time lapse seismic research in gas reservoir. Using Shapiro model to characterize dry rock modulus varying with effective pressure, and employing Batzle-Wang equation to describe fluid velocity varying with pressure, and jointing Gassmann theory to implement fluid substitution and describe velocity of fluid saturated rock varying with gas saturation, rock physics model of fluid saturated rock velocity varying with pressure and gas saturation is established. Based on the model, multi-wave time lapse seismic AVO (Amplitude-Variation-with-Offset) responses of gas reservoir model are simulated in different gas saturation and different effective pressure. Results demonstrate that effective pressure and gas saturation can be discriminated by multi-wave time lapse AVO technology and it is a guide to develop multi-wave time lapse seismic fluid monitor.