The characters of widening subgrade for Hu-Ning highway from three aspects of existing subgrade, new widening subgrade, conjoint section of the two subrogrades. The common problems in the engineering of widening subgrade were pointed out. The soft soil foundation treatment techniques, the choice of subgrade filling materials, the subgrade com- pactness controlling in the conjoint section and the application of geosynthetic were taken in- to consideration in order to guarantee the construction quality.