Synchronizer toothed ring is the key parts of automotive gearbox to shift.The synchronizer toothed ring performance is directly affects the smoothness in the process of shift and its quality of design is crucial. The development history of synchronizer is reviewed and the structure of three-cone synchronizer is introduced.The important part of design process for multi-cone synchronizer toothed ring-space verification is defined critically and its necessity is explained.The specific method and step of space verification is detailedly expounded.