A device for installing a snap spring in an upper sliding rail comprises an oiling device for oiling the inner side of an upper sliding rail body part with the U-shaped cross section, a snap spring installing device located on the lateral side of the oiling device, and a transferring mechanism transferring the upper sliding rail to the snap spring installing device from the oiling device. The snap spring installing device comprises a jig positioning the upper sliding rail, and a snap spring installing module located above the jig. The snap spring installing module comprises a first pair of clamping claws, a pair of wedge-shaped pressing blocks, and a second pair of clamping claws. The jig is movably provided with a pair of wedge-shaped driven blocks. During operation, the first pair of clamping claws clamp the front end of the snap spring, and the second pair of clamping claws clamp the tail end of the snap spring. The first pair of clamping claws move downwards firstly to clamp the front end of the snap spring into the upper sliding rail, then the wedge-shaped pressing blocks and the second pair of clamping claws move downwards, and the downward moving of the wedge-shaped pressing blocks is converted into the horizontal movement of the wedge-shaped driven blocks. The pair of wedge-shaped driven blocks transversely extrude the snap spring so that after the snap spring is compressed, the snap spring can be installed in the upper sliding rail from top to bottom.