The rapid development of Electric Vehicles (EVs) has had a prominent influence on the traffic system and distribution network. The charging load of electric vehicles is affected by quite a few factors and it has an important significance on modelling and forecasting of large-scale load. This paper firstly studies the impacts of each factor on charging power properties, including battery characteristics, SOC (State of Charge) and charging regimes, so as to guarantee the rationality of the input parameters of the power computation model and thus to establish the charging power computation model of a single electric vehicle. Then analysis is conducted on travel time, charging start and end time and vehicle flow properties, and the daily vehicle flow and traffic laws are studied for work days and non-work days based on the actual data of a practical expressway to establish the expressway EV charging load model. Finally, it is applied to a practical example using the Monte Carlo simulation method and the results verify the correctness and feasibility of the model.