The steam-accumulator, as a device for effectively saving energy, is widely used in industry. In the design of steam-accumulators, the quality of the released steam is analysed by means of the internal rule of boiler drums. It is found that this method is not trustworthy, because the evaporation process of an accumulator is different from that of a boiler. The pressure-dropping self-evaporation phenomena, which can be observed through a visible-system, are described in this paper. Based on the characteristic curves of the releasing-steam process obtained in a testing-system, the pressure-dropping self-evaporation process is analysed theoretically. Factors of various kinds affecting the pressure-dropping self-evaporation process are presented in this paper. In summing up it may be stated that the self-evaporation process inside a steam-accumulator can be categorized as pure surface evaporation, surface-intermittent boiling evaporation, and surface-continuous boiling evaporation,