To reduce greenhouse gas emissions in dealing with climate change, raplacing fule vehicles with electric vehicles (EVs) is one of the most effective measures. The multi-motor electric drive system (MMEDS), which is widely used in EVs, is driven by two or more permanent magnet synchronous motors (PMSM) by parallel shaft gear set (PSGS). Due to the action of forced synchronization of PSGS, once there is a rotational speed difference (∆V) between PMSMs, this will break the balance of output power between PMSMs and cause the increase in output power of some PMSMs while others decrease or even become negative, and eventually generate parasitic power. Parasitic power in MMEDS is bound to cause an autoexcitation vibration and a variation in dynamic characteristics of MMEDS. Therefore, in this paper, an electro-mechanical coupled dynamic model of MMEDS is set up, including a dynaimc model of PSGS and a dynaimc model of PMSM. Then, the affect of the forced synchronization action of PSGS on the dynamic characteristic of MMEDS is investigated under different ∆V. The results show that under the action of the forced synchronization of PSGS, the output power of the PMSM with a fast speed (or a slow speed) increase (or decreases) with the increasing of ∆V, and when ∆V ≤5 rpm, the increment rate or decrement rate of the output power of PMSM is obviously greater than that when ∆V>5 rpm. Meanwhile, ∆V has a singnificant effect on the nonlinearity of the vibration of the wheel, which increases first and then decreases with ∆V increases. The dominated frequencies of dynamic meshing force (DMF) of gear pair do not change with ∆V, but their contributions to the amplitude of the DMF change with ∆V. Moreover, the modulation frequencies also appear in the spectrum of the DMF, but their contributions to the amplitude of the DMF are singnificantly less than that of the dominated frequencies