A circularity detection tool for a pressure vessel comprises two parallel and horizontally arranged beams (1) and a connecting beam (2). Each of two ends of the connecting beam (2) is perpendicularly connected to one end of each of the beams (1). Each of the beams (1) is provided with two driving wheel elements (11) arranged in parallel. The circularity detection tool for a pressure vessel further comprises a detection arm (3). The detection arm (3) comprises a seat (31), a lower support arm (32), an upper support arm (33) and a detection head (34). The circularity detection tool for a pressure vessel, being provided with the detection arm (3) capable of conveniently adjusting a position of the detection head (34), is adaptable to cylindrical bodies having various diameters. The detection arm (3) is further provided with a pigment box (331), and a transmission belt (343) made of a water-absorbing material. Thus, a position of a dimension larger than an upper limit of a standard dimension can be easily marked
