The paper presents time-lapse seismic practices in YC13-1 and DF1-1, two offshore gas fields in South China Sea. The drained blocks and residual gas distribution of YC13-1 gas filed are predicted by time-lapse seismic data interpretation combined with core measurement. Based on YC13-1 time-lapse seismic study, a systemic time-lapse seismic study of DF1-1 Gas Field is conducted. In the study, several difficulties have been overcome: dataset matching in time-lapse data processing, calibration of time-lapse data with well information, and pressure decrease in blocks while the gas saturation and the gas water contact unchanged. The results are applied to update depletion plan, and the new data show it will get better recovery and better benefits.