Uncertain energy is characterized as randomness, fluctuation, and intermittency, which would bring in uncertain factors when connected to power grid. However, conventional probabilistic production simulation can’t deal with generation expansion planning and evaluation well in the above situation. In order to overcome these disadvantages, the concept of flexible production simulation for power system is proposed, whose general method and procedure are also discussed. Flexible production simulation can calculate the reliability and economic indices given by conventional probabilistic production simulation, and also provides capacity credit and avoidable cost considering uncertain energy and storage being connected to power system. Abandoned uncertain energy power could be evaluated by the theory presented in the paper from time scale and two aspects of peak-load regulation and frequency adjustment. In the respect of peak-load regulation, abandoned uncertain energy power is analyzed through the demand and capacity of peak regulation. Abandoned uncertain energy power in frequency adjustment level is assessed based on multi-scene analysis. Flexible production simulation for power system provides a theoretical basis for generation expansion planning and comprehensive assessment of the impact of uncertain energy connected to power system. The effectiveness and validity of the proposed method are illustrated by the simulation of 2020 Northwest Power Network Planning.