The brake system plays a key role in the vehicle safety system. The drivers usually cannot depress the brake pedal immediately in emergencies. A new vehicle brake-assist mechanism using the transmission principle of the clutch is designed to solve this problem. This mechanism can automatically assist drivers to handle the emergency when they failed to brake in time. Thus, the danger can be avoided initiatively. Through experiment, the reliability and effectiveness of the actuator is proved. Compared to manual brake, the designed mechanism can be more exact and rapid.