In order to better describe the dynamic interaction of high-order harmonics in Modular Multilevel Converters (MMCs), this paper models the MMCs based on Linear Time-Periodic (LTP) system theory, and performs a small-signal stability analysis of MMCs based on Floquet theory. First, the nonlinear time-periodic system model is established based on the bridge arm average model of the MMC, and then the model is linearized around the periodic orbit to obtain the LTP model of the MMC. Finally, the small-signal stability of the MMC is analyzed by the Floquet theory for the LTP system. The results show that the LTP model can accurately describe the small-signal dynamic response characteristics of the MMC; Floquet analysis can accurately portray the stability boundary composed of the MMC parameters. In addition, the model and stability analysis results are verified by electromagnetic transient simulation based on the MMC aggregation model. This work is supported by the National Key Research and Development Program of China (No. 2017YFB0903705).