During the practical researches and applications based on passive visual servo, weld torch was often moved to the weld seam initial position manually by human or by the means of accurate mounting devices, which will often cause low efficiency, as well as a increase in manufacturing cost. To solve this problem an algorithm was presented in this paper to identify the initial position of the Aluminum plate weld seam. It’s totally based on passive visual servo system without utilizing any auxiliary optical equipment. This algorithm combined a variety of DIP techniques and approaches, accurate initial position could be extracted successfully. More images were processed by this method to confirm the validity. With the realization of auto-identification precision of the weld jig can be reduced dramatically and the degree of the automation and flexibility can be improved greatly.