The utility model provides a combined positioning device for a side beam of a stainless steel car roof, comprising an inner cavity positioning block (2), an external support body (3), an external positioning block (4), a push rod (5), and clamps (6), a top side of the inner cavity positioning block (2) fits in with a top inner portion of the side beam (7) of the car roof, the external support body (3) is adjustably connected to a surface of the inner cavity positioning block (2), the external positioning block (4) is adjustably connected to a surface of the external support body (3), an elongated nut of the push rod (5) is integrated with the inner cavity positioning block (2), and the clamps (6) are used to clamp two edges of the side beam (7) of the car roof. With the present utility model, the side beam of the car roof can be supported by fitting the inner cavity positioning block in with the top inner portion of the side beam of the car roof, and sizes such as width and height can be adjusted through adjustments of the external support body connected to the inner cavity positioning block, the external positioning block and the clamps.