Abstract: The mechanical properties of soils undergo significant changes before and after liquefaction during an earthquake, which consequently lead to a qualitative shift in surface and structural damage mechanisms. Therefore, it is important to determine the liquefaction trigger time for grasping the dynamic evolution process of liquefaction disaster and liquefaction mitigation. In this study, a new method for detecting the trigger time of site liquefaction based on strong motion records is established. Based on the theoretical model of two-layer soil, the relationship between pore water pressure development, the site's basic period, and the frequency variation of surface acceleration is analyzed, and the basic principle of the method is clarified. The technical flow of the method was developed by quantifying the time-frequency characteristics of the acceleration records in terms of time-frequency analysis techniques and frequency decrease rate parameters. Using 24 strong motion records from actual liquefaction sites during 11 earthquakes worldwide, along with numerical simulations, the two established methods and the proposed method in this paper are compared and evaluated. The results demonstrate the advantages of the proposed method. The proposed method not only achieves higher accuracy, but its main control parameter is derived from the theoretical model, independent of empirical data, enhancing its generality and applicability. This approach overcomes the limitations of existing methods, such as their reliance on manual intervention, which hampers reproducibility, as well as their dependence on measured records, thereby facilitating broader applicability and implementation.