The invention relates to a direct-fired half-gasification downdraft type full-gasification multifunctional straw furnace, belonging to the technical field of cooking and warming equipment. The furnace is innovatively characterized by the invention of a duplex circulative purification device, wherein the device comprises a horizontal slope cross board, a vertical cross board, a staggered round hole and a round disk-type drier purifying mechanism. The furnace is further innovatively characterized by the design of a triple ash cleaning device which can clean furnace ash thoroughly, conveniently and fast. The straws of crops are utilized to produce a combustible gas, the combustible gas is circularly purified and filtered to remove impurities and tar and further enhance the quality of fuel gas and improve cooking and warming effects, thus, the environmental pollution is reduced, and the problems of the traditional gasification furnaces that the quality of the produced gas is poor, the emission of waste gas is too much, the furnace is not available during power failure and in case that the straw is wet, the tar is difficult in processing and the combustible gas has peculiar smell are solved. The straw furnace of the invention realizes stable gas production, good availability during power failure and in case that the straw is wet, good quality of fuel gas, no peculiar smell in the combustible gas, no pollution and the like, can be extensively applied to household cooking, water heating, warming and the like in urban and rural areas, and has extremely wide market prospect.