There exists a class of components in aerospace equipment products whose external surfaces are freetolerance dimensions but whose local or internal parts have accurate assembly requirements. The lack of suitable external surfaces to be used as accurate positioning datums has led to challenges in automated assembly and manufacturing of these parts. As a result, these parts have long been assembled using inefficient manual assembly methods. To solve this problem, this paper takes the automatic interference pressing requirement of a pin in a rudder body as an example to carry out the research on the automatic assembly positioning method. Based on the analysis of the traditional one-side two-pin positioning error and the finite element analysis of the influence of positioning eccentricity on the press fitting quality, a new automatic assembly technology is proposed, which takes the assembled feature itself as the benchmark for accurate positioning. The test results show that this method can realize the precise positioning of the pin quickly. The research work provides technical support for the successful completion of a high quality and efficient automatic assembly line for the pin of a certain rudder parts.