The back analysis principle and method for thermal conductivity of RCC is introduced in this paper, and a Jiang Ya RCC dam is cited as a building example , on the basis of prototype temperature data of dam, the mathematic model by Difference principle is founded, the thermal conductivity of RCC is obtained. The result is similar to the data which is received in the laboratory. The back analysis is proved credible. The back analysis for thermal conductivity, on the one hand, actual safety of the completed projects could be evaluated; on the other hand, the design and construction in progress may be optimized. the scientific foundation for optimizing the design and monitoring the running of dam is provided. And it is provided with some scientific value and practical significance.