The invention relates to a soft soil foundation heavy traffic road subgrade, which is characterized in that the overall thickness of a subgrade working area is 150cm to 200cm, and is sequentially provided with slag, steel-plastic grids, broken stone, lime soil, cement lime soil and cemented soil. A subgrade processing method under a soft foundation heavy traffic condition is determined according to the depth of the working area, the subgrade within the depth of the working area is processed by sequentially paving the slag, the steel-plastic grids and the broken stone from bottom to top (the thickness of the slag is 40cm to 60cm, the thickness of the steel-plastic grid is 1cm to 3cm, and the thickness of the broken stone is 20cm to 40cm), and then the lime soil which is 20cm to 60cm thick, the cement lime soil which is 20cm to 60cm thick and the cemented oil which is 20cm to 40cm thick are paved. The subgrade has the advantages of high strength, good stability, good duration and the like, and the expense of the road in the full-service-life period can be remarkably reduced. The invention relates to an environment-friendly road design method.