Ti-6Al-4V is a typical α+β titanium alloys. It presents the diversiform microstructures and thus various properties under different heat-treatment regimes and thermomechanical conditions. The thorough understanding of the formation mechanisms and the evolution rules of different microstructures is crucial to the optimization of the alloys. In this paper, the formation and the evolution of sideplates in Ti-6Al-4V alloy are simulated by using phase field method, with the thermodynamics and the mobility databases used as the input of the phase field model and the primary α phase assumed to exist at the grain boundary. The simulation results show that under a certain condition the grain boundary α phase can grow into β matrix and form sideplates, and that the morphology of α colony is related to the interfacial energy anisotropy. The grain boundary orientation has a strong effect on the sideplate structure. In addition, the heat-treatment temperature can also change the morphological features of sideplates. Higher temperature can slow down the growth of sideplates and result in wider sideplate spacing as well.