With the proposal of the new infrastructure concept, new loads represented by 5G base stations and electric vehicles are connected to the urban distribution network on a large scale, which has a significant impact on the safety and economic operation of the distribution network. In order to evaluate the capacity of urban distribution network to accommodate the new infrastructure load, a method for evaluating the available capacity of distribution network based on load growth demand and distribution network carrying capacity is proposed. The total amount of increased load of each load node is caculated as the available capacity under the condition of new infrastructure load and distributed energy access. Considering the constraints of voltage and power flow, the second-order cone relaxation method is used to solve the model. In addition, considering the influence of the location and sequence of distributed photovoltaic access on the available capacity, the grid-connected position of distributed photovoltaic is optimized according to the dynamic programming method with the objective of maximising the available capacity. The simulation analysis is carried out in a regional distribution network grid, and the results show that the proposed method can effectively evaluate the available capacity level of the urban distribution network grid, and realize the dynamic optimization of available capacity through the orderly access of distributed photovoltaic.