The key features for highway extension along river are occupation of the widening road on the original river, deep marine clay, and oblique intersection of deep marine clay with weathered rock layer. Therefore, the foundation is necessary to be reinforced. In addition, the opposite riverbank requires to be expanded and excavated which would have bad influence on the effect of ground improvement. Thus, modification method must be determined carefully. Based on numerical analysis with ABAQUS software and field measurements, the economy and reinforcement effect of the Bidirectional Dry Jet Mixing Method(BDJMM) and Concrete Prefabricated Square Pile are compared. Research results indicate that(1)the reinforcement effect of combination of BDJM and plastic drainage plate is better than that of traditional DJM method. (2)For the oblique intersection of soft clay layer with weathered rock layer, a significant sliding shearing interface at certain depth of the pile body can be formed. Concrete Prefabricated Square Pile would not meet with such kind of problem. So the Concrete Prefabricated Square Pile is the best choice in the ground improvement with the condition of oblique intersection of soft soil layer and rock layer.