Abstract: The startup process of LLC resonant converters is a complex dynamic process, and existing startup schemes often struggle to simultaneously ensure that the resonant current remains below the current limit and achieve a rapid startup. To achieve a fast and stable soft startup, this paper proposes a four-stage soft startup scheme for LLC based on state plane analysis. Through analyzing the state trajectory during the LLC startup process, the LLC startup process is divided into four stages. The resonant current at each stage is calculated to ensure that its maximum value does not exceed the current limit. Different implementation methods are designed for each stage, and the digital control implementation method of the LLC four-stage soft startup scheme is introduced. A simulation model and experimental prototype were built to verify the safety, speed, and the accuracy and rationality of the calculation of the four-stage soft startup scheme for LLC.