The invention discloses an online detection device and method for the air tightness of an in-service pressure vessel, which is used for carrying out air tightness detection on the in-service pressurevessel. The detection device comprises a base, a vertical rod, a folding rod a, a fixed rod, a pull rope, a motor a, a winding roll, a guide rod, a rotating wheel, a conveyor belt, a motor b, a tension spring, an infrared thermal imaging camera and an operation table; the vertical rod is vertically arranged on the base; the folding rod a is connected with the top end of the vertical rod through arotating shaft; the fixed rod is arranged on the folding rod a and the side surface of the joint of the folding rod a and the vertical rod; one end of the pull rope is connected with the fixed rod, and the other end of the pull rope is connected with the winding roll; the tension spring is arranged between the motor b and the base; an output shaft of the motor b is connected with a transmission wheel; the infrared thermal imaging camera is arranged on the conveyor belt; the operation table comprises a master controller, an operation panel and a display; and image analysis software is arrangedin the master controller. The detection device can adapt to a field complex environment; and a leakage point of the pressure vessel can be accurately and quickly analyzed by the detection method.