Abstract: A sustainable energy（new energy）penetration limit assessment method considering the frequency security of power system is proposed in this paper. In this method,the maximum frequency deviation expression of the system after renewable energy access is derived,and an approximate solution method for the extreme value point of power grid frequency deviation is given,and then based on the approximate results,the variation of the maximum frequency deviation of the power grid with changes of the inertia and the equivalent adjustment constant is evaluated under the capacity changes of new energy generating units. Furthermore,the frequency safety constraint model is established as the rigid constraint of sustainable energy penetration limit. Finally,the approximate model is tested on two test systems and the simulation results are compared. The simulation results show that the maximum new energy penetration level of the system as determined with the proposed method has high accuracy and can guarantee the stability and reliability of the system.