In order to fit the situation in the future that electric vehicles will be widely used, it's necessary to study the characters of the electric vehicles for transportation planning and management. As an important part of the traffic demand forecast, the traffic assignment model needs to be updated to fit the new characters of electric vehicles. Add distance constraint to original users equilibrium model and set different proportion for electric vehicles and gasoline vehicles, then the new users equilibrium model that is based on the distance constraint of the electric vehicles can be found. Make traffic assignment in the simulated road network and compare the results to those in the original users equilibrium model. The final results show that both the distance constraint and the proportion of electric vehicles in the whole network strongly influence the distribution of traffic flow.