The invention discloses an automatic assembling device for a railway cantilever assembly. The automatic assembling device for the railway cantilever assembly comprises a rack, a first clamping and feeding mechanism, a second clamping and feeding mechanism, an assembling and positioning mechanism and a bolt tightening mechanism, wherein the first clamping and feeding mechanism is used for clampinga flat cantilever pipe and feeding the flat cantilever pipe to the assembling and positioning mechanism, and a connecting sleeve is placed on a positioning and supporting groove so as to be positioned; and then the second clamping and feeding mechanism is used for clamping an inclined cantilever pipe to horizontally insert into the connecting sleeve, and the inclined cantilever pipe is tightened through the bolt tightening mechanism. The automatic assembling device can be used for automatically assembling the flat cantilever pipe and the inclined cantilever pipe, so that the assembling efficiency and the assembling precision are improved. In addition, the invention further discloses an assembling method using the automatic assembling device, so that manpower is replace, the assembling precision is high, and the efficiency is high.