With the integration of a high proportion of renewable energy, the fluctuations and uncertainties bring a higher demand on the flexible ramping capacity of the power system. This paper proposes an optimal dispatch of adjustable resources such as conventional generation units, energy storage systems and flexible loads of the power system for a flexible ramping product (FRP). First, an estimation method of the flexible ramping requirements considering the uncertainties of the day-ahead forecasting net load is proposed according to the start- and end-time net load and its safety margin to quantitatively estimate flexible ramping requirements. Second, based on the generated uncertain scenarios of the net load, it proposes an optimal dispatching model of the adjustable resources with a two-stage mixed-integer linear programming approach. The dispatching model is implemented from the day-ahead unit commitment and real-time economic dispatch. Finally, four typical cases are considered to compare and validate the optimal dispatching model based on the IEEE-RTS-24 bus system. The results show that the proposed dispatching model can effectively improve system flexibility and reduce operating costs.