The oil casing online monitoring system was successfully applied for the first time in Well Yan 21-P2 by Shaanxi Coalbed Methane Drilling Company. This system uses the density and gravity of water to prevent the leakage of gas or liquid, effectively detecting the tightness between casings. During the casing running operation, after each casing is run in, the monitoring instrument is immediately placed inside the top casing for pressure test detection. Only when the detection data is qualified can the operation of running the next casing continue. The application of this system has improved the efficiency of casing running operations in Well Yan 21-P2 and significantly reduced the safety risks and unnecessary workload caused by insufficient casing tightness.