The invention discloses a device for assembling and welding a fan casing combined piece. The device comprises a body assembly, a gas protecting assembly and a positioning assembly; the body assembly comprises a hollow disk; a plurality of bosses used for supporting a casing are arranged on the periphery of the hollow disk; the positioning assembly comprises a casing positioning assembly used for positioning the axial direction, the angular direction and the circle center of the casing, an inner ring positioning assembly used for positioning the axial direction, the angular direction and the circle center of an inner ring, a blade boss positioning assembly used for positioning the angular direction of blades, and a blade axial direction positioning assembly; the blade axial direction positioning assembly is arranged on the inner side of the hollow disk, and comprises a hinge plate; one end of the hinge plate is hinged to an overturning base, and the other end of the hinge plate is connected with a positioning block in a sliding manner; a first pressing device is fixedly arranged on the hinge plate; and a pressing head of the first pressing device is fixedly connected with the positioning block. The device is convenient to operate, fast and capable of saving labor, reducing the number of operators, and making blade welding kept consistent.