Abstract: When the train operation process was affect by emergency, the train couldn’t be fully guaranteed to operate with train diagram. The paper put forward the stability index of train diagram, introduced Cellular Automaton(CA) model to simulation of the train operation process, calculated the stability index of train diagram, took the Beijing-Shanghai High-speed Railway Line as an example to simulate the train operation, according to the actually train diagram, got the relationship among the emergency probability and the train maximum speed and train diagram stability, designed the train maximum speed in order to ensure the train diagram stability in the case of emergency probability, proposed a new analytical method for the train diagram stability, got maximum permissible speed in different conditions. It was with important significance for drawing up the train diagram and dispatching the train operation.