Several advantages are associated with an atmospheric microwave plasma torch, including low cost, easy processing, and high density of electrons and reactive species. This paper develops a compact microwave plasma torch (CMPT) operating at 2.45 GHz with an ultrabandwidth of 30 MHz. The structure and equivalent circuit of CMPT is established and analyzed based on theory of transmission line. The simulation results indicate that the resonant frequency will drift downward with the presence of plasma. In order to keep the better coupling between the microwave and plasma, a new design scheme of a fully automatic microwave plasma system is proposed. The effects of different microwave powers on the CMPT discharge are carried out on this system. Finally, in order to demonstrate the performance of the CMPT, an experiment to modify the surface of polymethyl methacrylate (PMMA) is conducted. It is found that the water contact angle is reduced from 85.4° to 32.3°, and the surface morphology characterized by atomic force microscope indicates significant improvement of the surface wettability of PMMA, which proves that the CMPT system is feasible and available.