Based on the theory of elastic layered system, this paper establishes a three-dimensional finite element model of cement concrete overlay structure on old cement concrete pavement with the Abaqus program, and analyzes the influence of modulus change of old cement concrete pavement, modulus change of composite foundation and various overlay thickness on the load stress at the most unfavorable position of different overlay bottom. The analysis results show that before microcracking the old cement concrete pavement, it is necessary to treat the diseases such as voids at the bottom of the old cement concrete slab to ensure the stability of the strength of the old cement concrete slab after microcracking. The strength of the old cement concrete slab after microcracking is not as high as possible, and the strength of the old cement concrete slab is the best at 4000 Mpa. The process of road reconstruction should be timely, the foundation should be reinforced, and its strength should be kept at 180 MPa; the recommended thickness of cement concrete overlay is 18 cm.