The California Bearing Ratio(CBR) denotes the potential strength of subgrade material and is the important index of evaluating its using performance in expressway. In order to study the main influence factors of the CBR value of clay, The physics and chemistry tests of fifteen groups subgrade materials, coming from the Qing(Dao)-Yin(Chuan) expressway in hebei province, are done. Based on the result of tests, which there are obvious difference between CBR value with the same plastic index clay under the same test method and condition, the influence factors of California Bearing Ratio of expressway subgrade materials are distribution of particle sizes in soils, mineral component and clay mineral component. The profitable conclusions are gained: the main influence factors of California Bearing Ratio of expressway subgrade material are the type of mineral component and the content of each mineral, the distribution curve shape of particle sizes in soils and the clay ratio silt secondly. The CBR value minishs with the clay ratio silt increasing.