The invention discloses an in-tube single weld seam pressure test device. The device comprises a pressure test plug and a hydraulic dolly for driving the pressure test plug to move in a tube; the pressure test plug comprises a front pressure plate, a pressure test main body, a pressure test pipeline, a hydraulic pipeline, a rear pressure plate, a hydraulic cylinder, a sleeve and a guiding cylinder; rubber sealing rings respectively sleeve two sides of the pressure test main body, the outer side of every rubber sealing ring is provided with a compacting ring, the front pressure plate and the rear pressure plate are respectively arranged at two ends of the pressure test main body through the guiding cylinder and the sleeve, and the front pressure plate and the rear pressure plate are respectively provided with extruding rings pressing against the compacting rings; and the hydraulic cylinder is arranged on the rear pressure plate, the end portion of the piston rod of the hydraulic cylinder is fixedly connected with the front pressure plate, a spring is arranged between the front pressure plate and the cylinder body of the hydraulic cylinder, and the pressure test pipeline traverses through the rear pressure plate and is connected to the pressure test main body. The device can carry out individual pressure test on an annular weld seam in the tube, and also can enhance the safety and the reliability of weld seam pressure test and increase the working efficiency of the weld seam pressure test.