A new quantification method on transience reliability for drivers is proposed by a new definition ＂confidence degree＂, whose error rate is calculated by the response time of drivers, and the influence factors of reliability for drivers in different running stage are analyzed in the paper based on Behavior-causing Theory. The simulation results show that the new method could not only statiscally analyze the reliability but also describe the relationship between reliability and time and it could analyze the influence factors to influence the transience reliability degree. And some advices for driving safely are given.