Abstract: At present, the access of distributed generation and electric vehicles with ultra-fast charging to the distribution network will increase the operational risk of the system. Aiming at the problem that the existing risk assessment indicators fail to consider the system capacity effectively, this paper proposed a comprehensive index of risk assessment of distribution networks considering the power system's capacity. The distributed generation output model and the electric vehicle load model with ultra-fast charging were first constructed. Then, the index based on the distribution entropy theory was used to measure the risk caused by the over-limit of state variables such as voltage and power and the uncertainty of their distribution. The index based on the Theil entropy theory was introduced to measure the risk caused by the equilibrium degree of the incremental distribution of state variables after the load impacted the system. The weight analysis method combining the order relationship method and the objective weighting method was used to evaluate the system risk comprehensively. Finally, the rationality of the comprehensive index was calculated and analyzed by testing the IEEE 33-node system. The results show that the comprehensive index proposed can effectively evaluate the potential risk when the system does not exceed the limit and consider that the ultra-fast charging of electric vehicles will change the time series distribution characteristics of comprehensive risk. The research in this paper can provide theoretical guidance for the safe operation of electric vehicles. with ultra-fast charging connected to the distribution network.