The invention relates to a device and process method for automatically controlling welding seam shape in longitudinal submerged arc welding. The device comprises a welding gun of which the front end is provided with a first image acquiring system, and a slag scrapping device of which the rear end is provided with a second image acquiring system, wherein the first image acquiring system is used for transmitting an acquired pre-welding groove image to a data processing system to process, selecting the optimal welding process parameters from a preset welding parameter database, and then performing welding and slag scrapping operation; the second image acquiring system is used for transmitting an acquired post-welding welding seam image to the data processing system to process so as to obtain shape parameters, comparing and analyzing the shape parameters through the preset welding parameter database, and welding the optimal post-welding welding seam and the corresponding welding process parameters; the welding process parameters are stored in the welding parameter database to be used as the optimal directive parameters for the next welding process; a curve-fitting manner in image processing enables quantization of the groove and the welding seam shape, and thus a basis for evaluating the welding seam is provided.