Abstract: In order to achieve real-time monitoring of reservoir parameters such as oil-water interface and online precise control of wellbore production, a post casing reservoir monitor system was researched. Based on transient electromagnetic theory, a post casing reservoir detection model based on line sources was established. Through numerical simulation, the basic characteristics of the post casing reservoir monitoring system and the influence of different factors on the detection performance of the post casing reservoir of line sources are analyzed. The simulation results show that using the bushing as a line source can apply a greater current, and the strength of the secondary field signal is positively correlated with the line source current; The suitable emission length of the line source is around 300 m; The larger the wall thickness of the casing, the higher the amplitude of the electric field response.The post casing reservoir monitoring system based on line source has good oil water interface recognition ability within a range of 15 m, which can provide effective technical means for online monitoring and stable oil and water control of oil and gas wells.