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 monitoring system based on line sources was established according to the transient electromagnetic theory. Through numerical simulation, the basic characteristics of the post-casing reservoir monitoring system and the influence law of different factors on the post-casing detection performance of the reservoirs based on line sources were analyzed. The simulation results show that using the casing 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. A larger wall thickness of the casing indicates a higher amplitude of the electric field response. The results show that the post-casing reservoir monitoring system based on line sources 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.