The invention discloses a self-locking clamp suitable for an ultralow-temperature steel bar tension test and a using method of the clamp. The self-locking clamp comprises a low-temperature test box, a steel bar test sample, an upper clamp and a lower clamp, wherein the upper clamp and the lower clamp are used for fixing the steel bar test sample in the low-temperature test box, the upper clamp and the lower clamp are connected with one end of the steel bar test sample respectively and are fixed on the low-temperature test box, one end of the upper clamp and one end of the lower clamp, matched with the steel bar test sample, are in hollow sleeve structures with U-shaped cross sections, the inner diameter of each U-shaped opening end is less than that of the bottom of the corresponding U shape, the inner wall of each hollow sleeve is in smooth inclined transition from the corresponding opening end to the bottom, and the self-locking clamp also comprises wedge blocks arranged inside the hollow sleeves. According to the self-locking clamp and the using method of the clamp, the upper clamp and the lower clamp, which are of the hollow sleeve structures with inclined surfaces, are arranged and are matched with the wedge blocks to fix the steel bar test sample, the cutting operation to the sample is avoided, and data of the steel bar test sample in the ultralow-temperature steel bar tension test are real and reliable.