Th invention discloses an automatic assembling device and an automatic assembling method for a welded steel bar truss deck, and belongs to the technical field of building material production equipment. The device comprises a rack, a feeding platform, a front feeding slide plate, a spot-welding structure, a back feeding slide plate, a positioning structure and a discharging platform, wherein the feeding platform, the front feeding slide plate, the spot-welding structure, the back feeding slide plate, the positioning structure and the discharging platform are sequentially arranged on the rack from front to back. According to the method, only a steel bar truss needs to be placed by manpower (1 to 2 persons) in the step (2), and other processes can be automatically controlled through the cooperation of an electric control cabinet (programmable logic controller (PLC) and other accessory circuits can be arranged inside, and the electric control cabinet is provided with the structure which isthe same as a common control structure), various touch switches, a control switch of an air cylinder, a control switch of a drive motor for a guide wheel and a guide roller, and the like, so that theoperation can be carried out simply and safely (no interference with an electrode), the efficiency is high, multiple steel bar trusses can be quickly assembled at the same time, the welding between the steel bar trusses and a soffit formwork is equidistantly realized, and the assembling efficiency can achieve more than 50m<2> / h. Automatic positioning and centering can be basically realized automatically, the operation is simplified, and the assembled product meets the requirement.