The part number in 2D assembly drawing combining with BOM (Bill of Material) is an important basis for the assembly of die parts. At present, the automated annotation of part number by 3D design software cannot achieve the vegular arrangement and statistics, and the labels are serious overlap, which need to be adjusted through manual operation that cost lots of time and energy. Therefore, an automated annotation algorithm of part number was proposed based on the equant circle, and the labels of part number were arranged around the assembly drawing according to the rules merging the parts with the same specifications to one label. Then, the algorithm was seamlessly integrated into the NX software platform, the assembly die parts of 3D model were correlatiuely recognized by 2D geometric features, and the labels of part number were extracted and marked at the algorithmic optimization location automatically. Finally, the automated annotation of part number in 2D assembly drawing were conducted, and the effectiveness and practicability of the algorithm and system were verified. The results show that the algorithm and system can annotate the part number automatically, and a variety of layouts and annotation styles can be also provided, which has the characteristics of high accuracy and efficiency.