In order to make different strength grades of concrete which meet the requirements of the complex engineering environment of China-Laos (Moding-Vientiane) Railway, the raw materials such as cement fabricated in Laos and water reducing agent made in China were chosen, the effect rules of size and the effect rules of different fly ash contents on the mechanical properties of C55 concrete were studied through the concrete mix proportion design and the mechanical property tests of different strength index. And the concrete mix proportion of C20∼C60 was determined. The results showed that the concrete which meet the requirements of China railway standards (TB10424-2010) can be configured according to the mix proportion in this paper. In the case of the additive is fly ash only, the appropriate content of fly ash in C55 concrete for China-Laos (Moding-Vientiane) Railway bridges was determined to be 15%. The test results in this paper had been successfully applied to the concrete construction of real bridges.