In the car-following process, there are some uncertain parameters in the vehicle brake system and the driver, such as the brake acceleration, the action time of the vehicle arrester and the response time of the driver, etc. Generally, the ranges of these uncertain parameters are easy to be found, but the effect of them on the car-following process is one of the hard issues. This paper used the interval number to describe the uncertain parameters in the car-following process. Based on the approximate computational method of the safety distance, the interval analysis was employed to calculate the safety distance of vehicles under two typical conditions. The results show that the safety distance calculated by the interval analysis method is not a certain value, but an interval value. Therefore, the interval analysis method can forecast the effects of the uncertain parameters on the safety distance in the car-following process.