The ultrasonically aided electrospray thrusters (UAET) are used mainly on micro-satellites (with mass less than 10 kg). In this work, numerical simulation studies of the UAET plume field are conducted to investigate the following two problems encountered during operational tests:the avertence angle of thrust direction, which exists between the design and test outcome, and the lower energy efficiency than the established theoretical value. In order to precisely model the special physical process of the UAET plume neutralization, we develop a new hybrid model named the neutralization of electrons and charged droplets for the plume fluid field to capture the neutralization process of electrons and positively charged droplets. This model describes the dynamical movement of particles, the collision between electrons and droplets, the breakage and coalescence of the droplets, and the flow and heat transfer between the droplets and background gas. To show the feasibility and accuracy of the model, experimental tests involving thrust measurements and high-speed photography of the plume are conducted. The comparison between the test and simulation results under the same study conditions shows that the average error of this model is about 20%, and both the test and calculation exhibit a consistent trend in the various study cases. According to this model, we simulate the plume fluid field of UAET (with 2-W discharge power and 2-mA current) and identify the distribution characteristics of several parameters, including the droplet number density, charge density and the droplet volume, as well as the energy consumption categories that occur. Our model can successfully demonstrate the internal mechanisms that cause the two problems identified above. Our work will provide support for future studies of optimal design.