LLC resonant converters have been widely used in engineering fields, and the stability of LLC resonant converters is still a research hotspot due to its variable frequency. In this paper, a time-domain stability analysis method based on Floquet theory is firstly proposed and applied to LLC resonant converter. Taking the half-bridge LLC resonant converter with PI controller as an example, its closed-loop time-domain model is established, and the stability of LLC resonant converter is following analyzed according to the eigenvalues of the time-domain model. Based on the analysis results, the stable range of the LLC resonant converter with different control parameters is obtained. Further, the accuracy and effectiveness of the proposed stability analysis method based on Floquet theory are verified by simulation results. Therefore, this paper provides a new method for the stability analysis of LLC resonant converters.