The invention provides a spiral pipe prewelding molding device. The provided spiral pipe prewelding molding device comprises a first molding backup roll (100), a second molding backup roll (200), a molding lower press roll (300), an external control roll (400) and welding equipment (500). Specifically, the first molding backup roll (100), the molding lower press roll (300) and the second molding backup roll (200) are distributed in order, and are all in rolling fit to a steel strip (600), the external control roll (400) is disposed below the steel strip (600) at one side away from the first molding backup roll (100), and is in rolling fit to the steel strip (600), the welding equipment (500) is used for welding the steel strip (600), and the welding position is located between the externalcontrol roll (400) and the second molding backup roll (200). The spiral pipe prewelding molding device provided by the invention can solve the problems of low quality and large labor amount of workers in existing prewelding molded spiral pipes mentioned in the background technology.