Based on Ansys software, a 3D model of multilayer Cu interconnect structure were established with three-level sub-modeling technology. The effects of elastic modulus and coefficient of thermal expansion of the dielectric material of the global interconnect lines in the 10-layers Cu interconnect structure on the thermal stress were studied. On these basis, the global interconnect lines dielectric material was optimized. The results showed that the coefficient of thermal expansion of the global interconnect lines dielectric material had less effect on the thermal stress of Cu interconnect structure, but the elastic modulus had a great influence. The thermal stress of dielectric material in each layer was proportional to the elastic modulus, while the interface thermal stress in SiN was inversely proportional to it. Finally, to reduce the thermal stress of the key positions of Cu interconnect structure, the selection of the global interconnect lines dielectric material was optimized through selecting different material combinations, then the reliability of Cu interconnect structure was improved.