Current research on flexible pavement distress control is focused on the structural layer, the operational condition of the soil foundation is neglected in most studies. The depth of soil foundation working area is crucial for the effective control of road distress. To investigate the variation patterns of the soil foundation working area depth, a three-dimensional finite element model of the road structure was constructed. The coupled effects of road surface irregularity and random vehicle dynamic loads were considered in the model.The effects of different factors on the soil foundation working area depth were analyzed. These factors included different grades of road surface irregularity, driving speeds, vehicle axle loads, as well as the dynamic modulus and thicknesses of various road structural layers. The results show that the soil foundation working area depth increases by 161.4% when the vehicle axle load is increased from 100 kN to 300 kN. When the roughness level of the road surface is decreased from grade A to grade D, the soil foundation working area depth increases by 14.4%. An exponential growth relationship is shown between the two factors. This trend is consistent with the impact of road roughness level on the random dynamic load of vehicles. However, the impact of driving speed on the soil foundation working area is relatively small. The soil foundation working area depth increases logarithmically dynamic with the increase of the soil foundation dynamic modulus. It decreases linearly with the increase of the dynamic modulus and thickness of each structural layer. The influence of each structural layer’s thickness on the soil foundation working area depth is greater than that of the dynamic modulus. When the soil foundation dynamic modulus is increased from 60 MPa to 260 MPa, the soil foundation working area depth increases by 52.2%. Within the conventional range of thickness and dynamic modulus values of each structural layer, the soil foundation working area depth is increased by 3.7%~24%. The results further indicate that the depth range of the soil foundation working area of flexible pavement is 0.88~2.34 m under the coupling of road roughness and vehicle dynamic load. Compared with the existing research depth range of 0.5~1.7 m, the upper and lower limits of the depth range are increased by 76% and 38% respectively. The research results provide a theoretical basis and technical reference for the depth of soil foundation reinforcement treatment under heavy traffic.