This paper researches the chaotic phenomenon of the permanent magnet synchronous motor (PMSM) in electric vehicles when the PMSM is in power on or power off and analyzes the chaos of the PMSM when parameters are within a certain range. Bifurcation diagram, phase portraits and Poincareé maps present traits of chaos in the PMSM. Finally, an effective way of controlling chaos is put forward by an external sinusoidal dither signal which is input to the PMSM when in power off. Some simulations are presented to demonstrate effectiveness of the method and to test what effect a sinusoidal dither signal of different frequencies has on chaos control of a PMSM.