Abstract: In order to improve the utilization rate of PWR nuclear fuel, a kind of uniformly mixed thorium-uranium assemblies which contain suitable quantity of ²³²Th and ²³³U were developed. Neutronics calculation and analysis show that k_inf of the new assemblies decreases with the increase of burnup slowly. This property is very good for extending reactor core cycle lifetime. Unit 1 of Ling Ao Nuclear Power Plant was chosen as reference core, and thorium-uranium mixed core was formed by feeding thorium assemblies. Through corresponding analysis, the conclusion indicates that uniformly mixed thorium-uranium assemblies suit exiting PWRs and have advantages in ²³⁵U utilization and long cycle lifetime was obtained.