CLLC resonant converters are gradually becoming one of the core topologies of bi-directional DC/DC converters due to their high power density, high efficiency, and low EMI. Different analysis and design methodologies have a significant impact on the performance of CLLC resonant converters. However, the traditional analysis and design is mainly based on a low-accuracy fundamental harmonic approximation (FHA). In order to obtain higher accuracy, this paper introduces the operation mode analysis (OMA), which can accurately predict the behavior of resonant variables and obtain accurate operation characteristics of CLLC resonant converter. Based on this, a detailed and programmed optimization design methodology is proposed, which can quickly shrink the design area. Then, the design space can be built by discretizing the area. The optimized design points can be found by comparing all the design points in the design space based on the loss model. Finally, a MATLAB graphical user interface (GUI) program tool is built to simplify the design process, compare device performance, and optimize design results. Since the tool has a much faster analysis speed than commercial simulation software, combined with the method of shrinking the design area proposed in this paper, the design speed can be greatly accelerated—only a few tens of seconds can automatically complete the entire design work, greatly improving the accuracy and speed of CLLC resonant converter design work.