With the continuous development of the IoT, compact wireless communication modules have become indispensable components, and their antennas are gradually being developed from external devices into onboard integrated devices. The serpentine antenna, a variant of the monopole antenna known for its small size and easy integration, is often applied to engineering practices. However, its performance has always been closely affected by the size of the surrounding grounding plane. By conducting a characteristic mode analysis (CMA), this study explored the variation patterns in the ground plane size and the resonant frequency. Based on the simulation results, it was clear that when the ground plane size is less than a quarter of the working wavelength, the ground plane will have a significant effect on the antenna’s resonant frequency. Thus, this study further analyzed a serpentine antenna with a grounding branch, and through analysis of the basic law of the influence of grounding structure on the antenna’s performance, we found that by adjusting the branch length, the matching performance of the antenna can be effectively improved. Furthermore, by changing the size of the ground plate, the antenna’s resonant frequency can be adjusted. Such a conclusion will hopefully provide a reference for future designs of integrated antennas in engineering applications.