The invention discloses an assembly device for pressure steel pipes. The assembly device comprises a fixed trolley, a movable trolley and a movable assembly trolley which are sequentially arranged on a rail; the fixed trolley is fixedly connected to the rail; the movably trolley and the movable assembly trolley are in sliding connection to the rail; the fixed trolley and the movable trolley are roll-welding trolleys; an assembly supporting mechanism and an operating platform are mounted on the movable assembly trolley; an assembly horizontal mechanism capable of horizontally moving on the assembly supporting mechanism and capable of being locked is arranged on the assembly supporting mechanism; the end part of the assembly horizontal mechanism is hinged with a circularity regulating stand column mechanism for regulating circularity of the pressure steel pipes; a steel pipe outer bracket is arranged below the circularity regulating stand column mechanism; a first hydraulic lifting jack is mounted on the steel pipe outer bracket; and the steel pipe outer bracket is positioned between the fixed trolley and the movable trolley. While used for assembling large joints of the pressure steel pipes, the assembly device can effectively reduce construction workers, can improve construction working efficiency, can improve site construction quality and can avoid damaging pressure steel pipe base metal by pressing and stacking.