Abstract: Drivers are used to steering, braking or steering-braking when they meet obstacles in order to protect lives and properties. Frecord data collecting system, dynamic GPS and sensors of braking force and angles are utilized to study the changing rules of drivers' braking velocitiy according to different time, the relation between maximum braking velocity and the distance between vehicle and obstacle, as well as driving velocity. The results show that drivers' braking operation includes four periods and also indicate that the drivers' maximum braking velocity is changing with distance in negative-power, however drivers' maximum braking velocity is changing with driving velocity in positive-linear. The regressive formulas are given. This research can help us to understand drivers' behavior under emergency and also offer theory basis for drivers' safety management.