Abstract: In order to study the influences of different gas saturation to the physical paramelers of the searm and to the AVO response fea-tures of the P-wave rellection wave ( PP wave) ,according to the fluid replaced principle ,a initial seam model was established and the physical paramelers of the seam under the condition of the different waler and gas phase to the saluration were calculated. According to the rellectance specrum theory a numerical simulation was conducled on the PP wave amplitude and the phase with the incident angle varied curve of the different gas content seam. The results showed that even if there were small content gas ( a volume fraction was 5%) in the matrix pores of the seam ,the P-wave velocity ,P- S wave velocity ratio ,Poisson's ratio and p- wave resistance would be obviously reduced. The reduced amplitude of each parameler would be 323 m/s ,0.397 0.062 and 620 g●m/( s●cm3) individually and the vibration ampli- tude of the PP wave would be reduced by 0.2 when the P-wave was in a vertical incidence. Therefore compressional wave velocity ,P-S wave velocity ratio ,Poisson's ratio and P- wave impedance could be the sensitive factors to detect the coalbed methane existed or not , meanwhile the PP wave A VO information applied to seek the enrichment zone of the coalbed methane would be feasible in theory.