The utility model relates to a movable hydraulic assembling tool for an under well pipe string and is used for the petroleum drilling engineering. The technical scheme of the movable hydraulic assembling tool for the under well pipe string is that the assembling tool comprises a movable platform with wheels, a hydraulic lifting mechanism located on the movable platform and a guiding mechanism connected on the lifting mechanism. The wheels are fixedly arranged under the movable platform via wheel racks; a lifting oil cylinder and a lifting piston rod are fastened on a pedestal of the hydraulic lifting mechanism; bearing sleeves are rotationally sleeved on the top part of the lifting piston rod on the guiding mechanism; the bearing sleeves are connected with a bracket; and two sides of the bracket are symmetrically provided with a roller wheel. Drill collars are assembled onto the tool; by hand pressing the button, up and down movements of the drill collars are achieved, so that axes of the two drill collars to be connected can be quickly aligned and buckling is avoided and a sealing surface is prevented from destroying; and the movable wheels on the bottom of the assembling tool enable the whole tool to move along an axis direction of the drill collars during the assembling of the drill collars, so that the assembling, the detection and the maintenance of the drill collars are facilitated, thereby guaranteeing safety of operators.