Abstract: The X gas-field is located in the western sag in southern part of East China Sea. The main gas layers are distributed in the sandstones with medium porosity and permeability, belonging to Mingyuefeng Formation of Palaeocene and with a depth range from 2 200 m to 2 800 m. M gas reservoir is normal reservoir, and the main gas reservoir of this gas filed. It has been confirmed through post-drilling AVO modeling and statistic analysis of drilling data that high-amplitude is highly related with distribution of gas reservoir. By forward modeling, a study has been done about the relationship between gas reservoir thickness and seismic amplitude, and the corresponding relationship between the distribution range of spot area with practical gas area. Through the above study, the forecast of gas reservoir and calculation of geological reserves of the gas-field can be done effectively.