Abstract: With the accelerated integration of renewable energy, power system operation faces increasing randomness and growing flexibility demand. Traditional production simulation methods are inadequate in characterizing the balance between flexibility supply and demand, and struggle to capture both temporal operation and stochastic characteristics. To address these challenges, a two-layer sequential-probabilistic production simulation model considering flexibility requirements is proposed. The upper-layer generates mid- and long-term generation plans considering unit dynamic constraints and power balance. The lower-layer captures the impact of uncertainties from both generation and load sides on system flexibility, based on representative net load fluctuation scenarios and multi-state flexibility supply models of generating units. Furthermore, the equivalent energy function method is employed to conduct probabilistic matching of flexibility supply and demand, calculate system flexibility indicators, and revise the upper-layer boundaries through a feedback mechanism. A year-long simulation is conducted on the modified IEEE 30-bus system to verify the effectiveness and feasibility of the proposed method. The results show that the proposed model can effectively evaluate the flexibility shortfalls and imbalance risks, optimize the allocation of flexible resources, and facilitate renewable energy accommodation and power supply security.