Abstract: The construction principle of the working brake device of trackless rubber-tyred vehicle is studied in depth. Under the condition of guaranteeing the constant braking force of the working brake device, a wet multi-disk fail safety type working brake device for trackless rubber-tyred vehicle was invented and designed. The device is a fail safety type working brake device, which controls the elastic force of the disc spring linearly through the hydraulic circuit, and brakes the trackless rubber-tyred vehicle and removes the brake by changing the dynamic and static friction disc tightness. The device not only has the working brake function, but also can play the fail safety type characteristics to automatically brake vehicles running out of control, to ensure the safety of trackless rubber-tyred vehicles and avoid major transport safety accidents.