The utility model discloses a self -adjusting drum brake, the purpose is when designing a service brake, but the cam automatic adjustment, it can both laminate with the brake drum to reach two shoe plate assemblies, drum brake that braking effect is better. Protruding axle hole design is adorned for the slot hole to the braking bottom plate, and two centres of a circle are at a distance of be 0.4 to 0.6mm, satisfies cam automatic alignment along about the two gyro wheel hole line of centres, causes two hoof assemblies can both hold the brake drum tightly, the category domestic initiation. The utility model discloses brake structure is simple, can be effectual the braking efficiency of improvement car, the hidden danger of having stopped partially about appearing when having reduced the vehicle service brake.