The invention relates to an automatic opening and locking mechanism for the carriage backdoor of a trackless rubber-tyred car. The automatic opening and locking mechanism comprises a specially designed hydraulic control system and the specially designed hidden carriage backdoor, and is mainly used for automatically opening and locking the backdoor of an undermine ejector dumping machine. The automatic opening and locking mechanism has the characteristics that the operation intensity of personnel is reduced, the misoperation is reduced, the working efficiency is improved, execute components are hidden on the inner side of a carriage, so that the execute components are protected effectively, and the automatic opening and locking mechanism is compact in structure,, and is firm and durable.