Abstract: It is necessary to design a reasonable market mechanism for the flexible resources which are urgently needed in the new power system. However, the cost characteristics of the energy constrained units, such as the energy storage or the reservoir hydropower stations, are significantly different from those of the traditional power sources. The spot market mechanism designed for the power system based on thermal power units is not suitable for the characteristics of the energy constrained units. Therefore, the characteristic differences of the energy constrained units are analyzed in this paper. It is found that when the scarce resources of the energy constrained units are changed from energy into power the cost is embodied in the form of energy rather than in the power. The necessity of building a new bidding mechanism for the characteristics of energy constrained units is demonstrated. On this basis, the bidding mechanism of the energy constrained units in the spot market is put forward, that is, changing from the power-based volume price curve to the energy-based volume price curve, a new clearing model of the spot market considering the energy constrained units is designed. In order to encourage energy storage to participate in the spot market, an energy storage capacity mechanism coupled with the spot market is also proposed in this paper. A numerical example verifies the effectiveness of the proposed mechanism and model.