The invention provides a device and method for detecting the effective spraying rate of the power spraying of the powder gun of a dry powder fire truck. The dry powder fire truck is parked on a weighing platform; mass sensors measure the axle load of each axle of the truck in real time and feed measured values back to a control device; and the difference value of the real-time measured values andthe total mass of the truck in initial full load is the mass of currently sprayed dry powder, and division operation is performed on the mass of the currently sprayed dry powder and dry powder injection time, so that the effective spraying rate of the powder gun of the dry powder fire truck under a current state can be detected in real time. The device of the invention can be combined randomly according to the number of the axles of the truck, and therefore, the device is convenient to carry and transport. With the device adopted, a plurality of times of weighing can be avoided, and detectionefficiency can be greatly improved. The device is just slightly affected by human and environmental factors, and can be an important detecting device for detecting the effective spraying rates of thedry powder guns of dry powder fire trucks or other dry powder injection systems.