The invention belongs to the field of porous ceramic materials and particularly relates to a gradient-pore silicon nitride and silicon carbide combined film tube and a preparation method therefor. The film tube material is prepared from silicon nitride and silicon carbide, and a gradient filtering structure is formed by a supporting body layer and a surface film layer; a supporting body is formed through combining silicon nitride produced from a reaction with coarse silicon carbide grains and has the average pore size of 10 to 80 microns, the surface film layer is formed through combining the silicon nitride produced from the reaction with fine silicon carbide grains and has the average pore size of 0.1 to 20 microns, and the overall porosity of the film tube is 35% to 50%. The preparation method sequentially comprises the steps of carrying out batching, forming the supporting body, preparing the film layer and carrying out firing, wherein the forming is carried out by adopting isostatic pressing, the forming pressure is controlled to 40MPa to 150MPa, the firing temperature is controlled to 1,400 DEG C to 1,650 DEG C, and the heat preserving time is 3 to 5 hours. The film tube can be used in an oxidative atmosphere, can also be used in a reducing atmosphere, is high in acid-alkali corrosion resistance and can be applied to the filtering and purifying of various high- and low-temperature fluids of coal gasification chemical industry, IGCC and PFBC coal-gasified power generation, high-temperature flue gas, automobile exhaust gas, water purification and the like.