The invention discloses a light-weight brake. An elastic reset mechanism is arranged in a window of an outer hub of the brake; a piston is arranged in an oil cylinder, meshes with inner teeth of the outer hub through outer teeth, and can axially move along the inner teeth on the outer hub of the brake so as to compress a spring reset mechanism; when oil is filled in the oil cylinder, the oil cylinder pushes the piston to move along the direction toward a friction plate so as to tightly press the friction plate and a steel disc, the same rotate speeds of an inner hub and the outer hub are realized, the spring reset mechanism is compressed to the shortest at the moment, and the surface pressure of the friction plate and the steel disc is equal to a difference between the control pressure ofthe brake oil cylinder and a return force of the elastic reset mechanism; and when the oil cylinder discharges oil, the spring reset mechanism pushes the piston back to the brake oil cylinder along the inner teeth on the outer hub of the brake, the friction plate and the steel disc are separated at the moment, and the inner hub and the outer hub ass well as the friction plate and the steel disc have relative rotation speeds. According to the light-weight brake provided by the invention, the weight power density of a speed change system can be improved, and the use demand of a planetary speed change mechanism is met.