<p>Underground gas storages (UGS) are an important guarantee for national energy strategic reserves, but the monitoring of gas reservoir distribution has always faced challenges. Time-lapse microgravity monitoring technology can infer the movement patterns of substances based on density changes at different times. Simulation results indicate that this technology provides strong support for the dynamic monitoring of UGS. However, in the process of processing time-lapse microgravity data, it is necessary to use field separation technology to obtain the gravity anomaly of the target body. In order to obtain more accurate and stable field separation results, this paper utilizes the low-rank nature of the regional field and the sparsity of the local field in potential field data, and adopts a method based on Robust Principal Component Analysis (RPCA) for field separation processing. In the study of the gas injection process in the Y21 UGS, microgravity measurement and processing results show that the areas with enriched natural gas in the UGS are approximately annular and located in the structural high-point areas, which basically match the geological structural characteristics. Due to the presence of boundary faults, according to the results of time-lapse microgravity, it is inferred that groundwater moves towards the structural high-point areas, and natural gas mainly moves towards the southwest direction, providing the direction of underground fluid movement during the gas injection process in the UGS.</p>