A new method on determining the dead zone of the bottom door mechanism is proposed in the premise of considering the friction. This method can ensure the bottom doors of the railway cars with the self-unloading coal hoppers to realize the prescriptive mechanical action accurately, timely, and coordinately. The effectiveness of the theoretical analysis results is verified by the computer simulation. This research provides the necessary theoretical reference not only for the application and maintenance of the existing bottom door mechanism, but also for the design, analysis and forecast of the new mechanism. It also lays the base for the motion reliability analysis of the mechanism.