The utility model relates to the technical field of dump trucks, in particular to a rear compartment door locking device of a dump truck, which comprises a truck body, a cargo compartment and a self-locking mechanism, the cargo compartment is pivoted to the truck body, a rear compartment door is arranged at the rear end of the cargo compartment, and the upper end of the rear compartment door is pivoted to the cargo compartment; the self-locking mechanism comprises a first latch hook and a damping telescopic rod, the first latch hook is pivoted to the cargo compartment and matched with the rearcompartment door, and a crank is fixedly arranged on the first latch hook; the damping telescopic rod can stretch out and draw back, the damping telescopic rod can generate force for preventing the damping telescopic rod from retracting back when retracting back, and the two ends of the damping telescopic rod are connected to the vehicle body and the crank in a pivoted mode respectively. By meansof the dumper rear door locking system, it is guaranteed that the rear compartment door can be opened when the cargo compartment is overturned for discharging, cost is low, and the failure rate is low.