Abstract: In order to solve the crack of precision forging process of tooth ring of manganese brass synchronizer at the corner of convex key, the microstructure of crack specimen was observed by metallographic microscope. Numerical simulation was used to simulate and analyze the precision forming process of the whole ring.The causes of microcracks were analyzed from the point of material and process of forgings, and the solutions were explored. The results show that the main reason of the crack is the large stress and strain in the forging process at the fillet. Adjusting the forging temperature can eliminate the microcracks produced by the forging process.The conclusion was verified by engineering in actual production.