Based on the one-part tariff system and the traditional two-part tariff system， this paper considers the dynamic ancillary capacity and introduces a novel two-part tariff system. The capacity is divided into the generation capacity and the dynamic ancillary one， which are squared separately. The trade center can procure enough ancillary capacity at less expense from a single market. Furthermore， taking the morrow on-grid competition of a certain power plant as an example， the success degree of capacity（SDC） is introduced to the model， and the optimal dynamic ancillary capacity is resolved with an algorithm of decision theory. Simulation results show that the proposed mechanism can compensate properly the ancillary services that units take and make them profiting.