The invention relates to a self-tightening round tube hoop tension testing device capable of keeping the curvature of a round tube. The self-tightening round tube hoop tension testing device comprisesa first separating disc, a second separating disc, a third separating disc and a fourth separating disc which are distributed along the circumferential direction of an inner ring of the round tube. Acylindrical structure matched with the inner wall surface of the round tube is formed by an outer ring obtained by splicing the first separating disc, the second separating disc, the third separatingdisc and the fourth separating disc; the first separating disc comprises a first clamping part opposite to the second separating disc; the second separating disc comprises a second clamping part opposite to the first separating disc; the first clamping part is matched with the third separating disc and the fourth separating disc; the second clamping part is matched with the third separating discand the fourth separating disc; and when the first separating disc is far away from the second separating disc, the third separating disc and the fourth separating disc are driven to move towards thecenter away from the round tube. The tension testing device is convenient to mount; during work, all the separating discs are self-tightened; and in the tension process, the curvature of the round tube is greatly kept, the influence of bending stress on the round tube is avoided, and further the accuracy is ensured.