The invention discloses a power supply system of a heavy-duty car SCR system. The power supply system comprises a DCU, a urea pump, a urea nozzle, a storage battery, a battery main switch and other electric equipment and further comprises a relay and a controller controlling the relay. A timer is arranged inside the controller. The DCU, the urea pump and the urea nozzle are connected in parallel, and then the power supply pin ends of the DCU, the urea pump and the urea nozzle are connected to a positive electrode of the storage battery through the relay. The power supply pin end of the other electric equipment is connected to the positive electrode of the storage battery through the battery main switch. The relay and the battery main switch are connected in parallel. The DCU, the urea pump, the urea nozzle and the other electric equipment are connected in parallel, and then the ground pin ends of the DCU, the urea pump, the urea nozzle and the other electric equipment are jointly connected to a negative electrode of the storage battery. After the battery main switch is turned off, it can also be ensured that the DCU, the urea pump and the urea nozzle are delayed to be powered on for a certain period of time, and urea backflow is controlled to be completed. The power supply system is simple in structure, high in reliability and suitable for large-scale application and popularization in the field of heavy-duty cars, circuits of the finished car are changed slightly, and the heavy-duty car can be conveniently refitted and upgraded.