Abstract: This paper proposes a margin assessment and security check method for new energy after grid connection. It aims to improve the efficiency of grid connection assessment of new energy and the safe operation level of the system. Based on the basic data, a new energy grid connection margin evaluation model is built considering the constraints of unit startup and shutdown time, unit output limit, rotating reserve capacity, secondary frequency regulation capacity, etc. According to the model, the unit operation plan and maximum access capacity are obtained. Then, combined with random sampling technology and unit operation plan, the optimal power flow model with correction control is constructed. And the new energy output prediction scenarios are substituted into the model one by one for verification. Then, according to the verification results, the upper and lower bounds of the installed capacity are gradually reduced by using the step-by-step approach strategy until the convergence conditions are met. And finally obtain the maximum safe access capacity of new energy. It is used for planning and construction of new energy power stations and provides guidance for orderly access of distributed new energy and coordinated planning of network sources.