The utility model is a unidirectional braking type brake for braking a trailer, which consists of a cylinder, a prepositioned brake shoe assembly, a postpositioned brake shoe assembly, a sliding dowel pin, a return spring and a brake plate assembly. The prepositioned brake shoe assembly has a big and a small brake shoe tendons, a brake shoe plate and a friction plate wherein the friction plate, the brake shoe plate and the small brake shoe tendon are connected as one body. The big brake plate is provided with a sliding waist-shaped hole. The big and the small brake shoe tendons are movably connected by the sliding dowel pin that passes through the waist-shaped hole and can move relatively. When an automobile or a tractor brakes, the trailer is capable of inertia braking in advancing direction. The automobile and the tractor automatically release braking of the trailer in the process of retreating.