Abstract: The Global Navigation Satellite System (GNSS) can be used to provide real-time and high-precision train positioning services. The application of GNSS-based train positioning technology can help reduce the trackside equipment of the train operation control system and simplify its onboard equipment, which is conducive to reducing the operation and maintenance costs of rail transit. For rail transit lines with complex terrain and numerous tunnels, there may be GNSS signal blind zones within which the GNSS cannot provide reliable train positioning data continuously for running trains, affecting the application of GNSS based train positioning technology. Therefore, the article adopts software defined radio technology to develop a train positioning simulation system for GNSS signal blind zones in rail stransit lines. The simulation system can generate train positioning simulation signal for GNSS signal blind zones in the line, so that trains can obtain accurate train positioning data when passing through GNSS signal blind zones, which is conducive to expanding the application scope of GNSS based train positioning technology. After building a testing environment in the laboratory, the system is run to generate static train positioning simulation signal for a single point(i.e.a station) and dynamic train positioning simulation signal for a section, and compare the simulation signals with satellite positioning signals collected on site to verify the feasibility and effectiveness of the system.