The invention discloses a device and a testing method for reinforcing steel bar tensile tests in a supper-low temperature environment. The device comprises a universal tester and a low-temperature box arranged between upper and lower jaw seats of the universal tester, wherein holes are respectively formed in the upper and lower surfaces of the low-temperature box, wherein the holes correspond to each other, and a connecting clamp which is connected with two ends of a tested reinforcing steel bar can penetrate through the holes; the end of the connecting clamp, which goes out of the holes, is fixed by the jaws connected with the universal tester; and the connecting clamp is threaded connection with the tested reinforcing steel bar. The device adopts the special connecting clamp; in a test, the connecting clamp can be firmly connected with the tested reinforcing steel bar, and the other end of the connecting clamp goes out of the low-temperature box and is connected and fixed by the jaws of the universal mechanical tester at normal temperature, so that the testing method is simple, the operation is convenient, the high efficiency is ensured, the purpose that the tested reinforcing steel bar is completely put in the low-temperature environment to be tested is reached, and parameters of the reinforcing steel bar, such as the low-temperature tensile resistance diameter shrinkage and the low-temperature tensile elongation, can be precisely tested; therefore, the device and the testing method can be widely applied to reinforcing steel bar tensile tests in the low-temperature environment.