In order to objectively evaluate the bearing capacity condition of the existing subgrade and to eliminate the deviation by the popular back-calculation software due to the initial parameter values.A finite element dynamic model of asphalt pavement was established and optimized the bending basin parameters based on correlation analysis screening, so as to propose a three-parameter subgrade modulus prediction model based on BDI-F₂-d₉. On this basis, a new method was developed to conduct a multi-layer pavement modulus back-calculation, and the effectiveness and accuracy of the model were verified by using the measured data of four different typical pavement structures of the Beijing RIOHTrack. The results showed that there is a good linear relationship (R²=0.910 6) between the predicted subgrade modulus by proposed FWD three-parameter model and the measured value by the bearing plate. The ratio of them is between 0.19 and 0.28, which is consistent with the literature findings. Since this predicted subgrade modulus with more precision is utilized as an input parameter for the dynamic finite element model of asphalt pavement to simulate the real dynamic process of FWD testing, the more accurate results of the modulus of each layer of the pavement will produced by back-calculating. The research results provide the reference for the initial parameters value of modulus back-calculation software and provide the theoretical basis for back-calculation methods to calculate subgrade and each pavement modulus using DBP indexes as well.