The synchronizer is a key component of the clutchless automatic mechanical transmission (AMT) for electric vehicles, but the inertial lock-ring synchronizer (ILRS) commonly used there is not very suitable for pure electric vehicles. To realize the rapid and smooth shifting with little impact, a new synchronizer named pressure-controllable friction ring synchronizer (PCFRS) is researched. Initially, the mechanical characteristics of the new synchronizer are analyzed, and the advantages over the ILRS are reflected. Furthermore, the dynamic transmission formulation based on the working mechanism of the PCFRS is established. Finally, the shift effects of the two kinds of synchronizers are simulated and compared. The research shows that the PCFRS can meet the main shifting evaluation index of an AMT without complex control methods, as well as it takes only 0.2406s to finish the comfortable shifting-no speed difference shifting. The shifting quality of PCFRS is better than that of the ILRS. It lays the foundation for the use of the new synchronizer for the clutchless AMTs of electric vehicles.