The great proliferation of renewable energy generation worldwide has brought about severe challenges to system planning and operation, i.e. the increase of operating risks. Due to the volatility and uncertainty of wind/or photovoltaic (PV) power, a switch to probabilistic methods which could handle uncertain variables has been advocated. In this paper, Authors propose a stochastic risk assessment approach to give a comprehensive evaluation of system security considering frequency regulation under high penetration of wind/PV generation. Authors present a new way of evaluating the security for RES-integrated power systems with the consideration of primary frequency regulation. The assessment relies on an improved cumulant-based PLF method using the multi-linearized model. For fast identification of operating limit violations, authors develop a cumulant-based procedure to get the overall power flow information. In addition, multi-linear processing is employed to reduce the truncated error caused by linearized load flow model for accuracy enhancement. The proposed risk model fully considers the likelihood and consequences of limit violation events by quantifying associated risk indices. Authors demonstrate the salient feature of the proposed method on a modified IEEE 57-bus system and its prospect for on-line application.