A new method of Widening Mountainous Highway with Integer Cantilevered Structures is proposed for steep mountainous areas after comparative analysis of the current methods used to wide roads. The design idea is that widening the road with integral casting wallpill retaining wall, making full use of exiting stable roadbed. The work principle of the integer cantilevered structures is analyzed, and successfully used in engineering practice. According to the engineering practice, this method which is applied for Widening Mountainous Highway with high and steep slope is not only simple in construction, economic and on the safe side, but also protecting environment