To address current challenges of difficult standardisation of dynamic determination of roadbed bearing capacity and excessive deviations in deflection control, this paper proposes a improved measurement method for roadbed load-bearing capacity based on the dynamic modulus control using a portable falling weight deflectometer (PFWD). The detection range of PFWD is determined by theoretical analysis. Based on the range, a finite element numerical model was developed, the spatial distribution and attenuation of the dynamic stress of the roadbed soil in the depth and horizontal directions are determined. The variation of the vertical dynamic stress with depth is not characterized by a gradual decrease, as is commonly believed, but rather by an increase followed by a decrease. The underlying reasons for the difference between static and dynamic indicators are explored. Using the improved method, a prediction model with dynamic bending as the detection index and static rebound modulus as the control index was developed. The proposed model can improve precision accuracy as well as detection efficiency, verify the effectiveness of the improved method, which is important for accurate dynamic measurement and evaluation of highway roadbed bearing capacity.