The invention relates to a composite marsh gas fermenting tank for marsh gas fermentation. Marsh gas fermenting tanks in the prior art all need external power settings; heat generated by biomass during the process of aerobic fermentation is not utilized to improve fermenting temperature of the marsh gas fermenting tanks; moreover, pre-treatment mode of solid-liquid separation of raw materials is generally adopted, thereby input cost is also increased. The invention consists of a stack retting tank provided with a heat exchanger and an internal circulating marsh gas reactor, wherein, a heat absorbing plate is arranged inside the stack retting tank; a heat exchanger is arranged on a reflux pipe in the marsh gas reactor; a closed circulation loop is formed by the heat absorbing plate and a pipe of the heat exchanger; circulating liquids are filled in the circulation loop. Therefore, the whole structure and the whole process do not need any external power, simultaneously marsh gas solid raw materials are simultaneously pretreated by the stack retting tank during the process of stack retting, thereby the contradiction of feeding of the solid raw materials and slag tapping is solved and simultaneously the cost is reduced; the invention greatly improves marsh gas application conditions in low temperature areas.