The utility model relates to an assembly tool, in particular to an automatic transmission bearing assembly tool. The automatic transmission bearing assembly tool comprises a positioning head and a baffle; the positioning head comprises a positioning disc and a positioning column positioned at the axis of the positioning disc and used for positioning a main shaft of a transmission; the positioning disc is fixedly connected with the baffle by a fixing rod; one end, away from the positioning disc, of the positioning column is provided with a push plate; one side, away from the positioning disc, of the push plate is provided with a push nut in threaded connection with the positioning column; one end, away from the positioning head, of the baffle is provided with an ejector plate abutted against a bearing; a plurality of ejector rods are arranged between the ejector plate and the push plate; the ejector rods penetrate through the positioning disc and the baffle. The automatic transmission bearing assembly tool has the beneficial effects of assembling the bearing under the condition that one end is not fixed, preventing bearings at two ends from being damaged due to press, ensuring uniform stress during bearing assembly, realizing coaxial assembly and the like.