Abstract: The detail information exchange and the process of the level transition from CTCS-2 to CTCS-3 were expounded in this article. The simulation platform of the CTCS-3 was taken as the research object. Because the existing system was lack of the level transition function, the level transition controlling module was added into the train-mount system to improve relevant functions of the ground system. The simulation test showed that the simulation platform could correctly complete the level transition from CTCS-2 to CTCS-3, implement the function of speed protection and driver confirmation in the process of level transition, record and display the simulation message between the train and ground equipment.