The invention provides a assembling device for assembling a cylindrical shell section and a seal head. The assembling device comprises a lifting appliance mechanism, welding robots, a rotary supporting platform and an external-clamping tool, wherein the lifting appliance mechanism is arranged above the rotary supporting platform so as to hoist the cylindrical shell section or the seal head; an internal rounding mechanism is arranged on the rotary supporting platform and is used for rounding the cylindrical shell section so that the cylindrical shell section can be fixed to the rotary supporting platform, and the cylindrical shell section and the seal head can rotate along with the rotary supporting platform; the external-clamping tool is arranged outside the rotary supporting platform in asleeving mode and clamps the outer wall of the cylindrical shell section and the outer wall of the seal head tightly; and each welding robot is arranged on one side of the rotary supporting platformto weld a girth weld between the cylindrical shell section and the seal head. According to the assembling device for assembling the cylindrical shell section and the seal head, the seal head is guidedand assembled through the external-clamping tool, assembling between the cylindrical shell section and the external part of the seal head can be realized, so that misalignment between the cylindricalshell section and the seal head during assembling or welding is avoided, workers do not need to climb to a workbench for operation, the labor intensity of the workers is reduced and the production efficiency is improved.