A soft rock is generally prohibited to be used as filling material for highway subgrade. A novel method for improving the performance of subgrade is proposed. Aiming at the large amount of highly weathered soft rock along Shiyan-Tianshui superhighway, the physical and mechanical properties of the natural soft rock is analyzed. Then the cement is mixed into the soft rock as filling material to better the subgrade performance.The field and indoor experiments showed that the maximum dry density and optimum water content increases when the cement content increases. By mixing the cement, the subgrade performance can be significantly improved.