Abstract: The California bearing ratio (CBR) reflects the potential strength of subgrade material and is an important index to evaluate its performance in expressway. In order to study the main influence factors of the CBR value and CBR characteristics of phyllitic slate, the CBR tests of the subgrade with materials of weathered soft rock from the Wu-Guang Railroad Passenger Dedicated Line were done. On the basis of the result of tests, the influence factors of CBR of the subgrade materials were studied，such as mineral component, immersion time, maximal particle size and striking times, and then a prominent property analysis was made with a method of nonrepeative double factor variance analysis, and useful conclusions were obtained that the main influence factors of CBR of the subgrade with materials of phyllitic slate are the type and content of mineral component, immersion time and striking times, and the influence of particle size was not very important. It is worthily emphasized that, these weathered soft rock can not be directly used as filling material in the immersion area and rainy region, the material must be further improved, and some measures, such as the protection of road slope and drainage of foundation face, must be adopted.