Abstract: Distributed photovoltaic（PV） is one of the important power sources to build a new power system with new energy as the main body. The rapid development of distributed PV has brought severe challenges to the access and consumption of the distribution network. As the most extensive access point of distributed PV, the distributor transformer faces great access pressure. This paper discusses the method of calculating the open capacity of distributed photovoltaic access distributor transformer. Firstly, the coupling characteristics of photovoltaic output and load are analyzed, and the coupling evaluation coefficient of typical electricity transformer is proposed. Then,based on the voltage constraint and the economic load of the distributed PV distribution, the open capacity of the distributed PV distribution was analyzed, and the security of the distributed PV distribution was verified. It will provide reference and basis for the planning, construction and operation of distribution network and the orderly development of distributed photovoltaic, which is of great significance for the construction of new power system.