Abstract: In the study of freight train departure mode,the research on the minimum number of cars in the fixed-point departure mode is a key issue.On the condition of some hypothesis of the freight train departure mode,by the analysis of car profits and locomotive loss in the course of car marshalling and train operation,the paper constructs a theoretical formula of determining the minimum number of cars in the fixed-point departure mode.And by this formula,the paper gives the change law of the minimum number of cars with the change of the time that the train runs in the railway sections and the change of the vehicle flow.And the change law is verified by the method of formulation.