A high/low temperature fatigue test fixture suitable for a round bar fatigue specimen comprises extension rods, split type chucks, opening clamping rings and bolts; clamping ends of the extension rods are connected with hydraulic chucks of a fatigue test machine; a blind hole is formed in the center of the end face of a specimen connecting end of each extension rod; six threaded holes are uniformly distributed at the periphery of each blind hole; each split type chuck is composed of two semicircular metal plates, semicircular central holes are respectively formed at the opposite sides of the semicircular metal plates, three through holes are uniformly distributed at the periphery of each semicircular central hole, and all the through holes correspond to the threaded holes of the extension rods and are used for connecting the specimen connecting ends of the extension rods to position and clamp the round rod fatigue specimen; the end ports of two ends of each opening clamping ring are each provided with a through hole, and the opening clamping rings are fastened by the bolts. The fixture has the advantages of simple structure, easy production and processing, convenient clamping and wide use, and also can be used for high/low temperature static tensile tests.