The invention discloses a fixture for assembling a protection sleeve on a U-shaped rod. The fixture comprises a workbench, a fixed seat arranged on the workbench and used for fixing one end part of the U-shaped rod, a slide seat capable of horizontally moving, a mandrel arranged on the slide seat and matched with a central hole in one end of the protection sleeve, and a clamping structure arranged on the slide seat, wherein an air inflation hole is formed in the centre of the mandrel and connected with an air pump through an air inlet pipe; and the clamping structure comprises clamping arms located at the two sides of the mandrel, and the end part of the protection sleeve is fixed through the matching of the clamping arms and the mandrel, and the protection sleeve is in sealing fit with the mandrel during clamping. According to the fixture disclosed by the invention, one end of the U-shaped rod is fixed through the fixed seat; one end of the protection sleeve is fastened by virtue of the matching of the mandrel and the clamping arms; after the U-shaped rod is connected with the other two end parts of the protection sleeve in a sleeving manner, a high-pressure gas is charged in the protection sleeve to inflate the protection sleeve, and the protection sleeve is pulled to be assembled onto the U-shaped rod as a whole; and the operation is simple and convenient, and the labour intensity is lowered.