Abstract: Based on the field survey of road traffic data, micro-simulation software VISSIM was calibrated and validated.Impacts on the capacity of exit were analyzed, and the velocity and volume of exit was utilized as evaluating indicators to study the varying laws of extra lanes in the case of different traffic flow combination.The outcome show that the length of extra lane is concerned with the traffic flow of the exit and the secondary road, and the reasonable length is recommended as 100~120 m.