The landslides of expansive rock cutting slope was the most serious and difficultly treating engineering problem for highway construction. The K9+600-K10+800 section of Beijing Western Sixth Ring highway was an expansive rock deep cutting slope, the problem of which was firstly encountered for Beijing highway construction. In this paper, the causes and characteristics of expansive rock slope failure were analyzed based on the engineering geology and rock properties of this section. In addition, the geogrid reinforced flexible supporting treatment measure was presented and its design scheme and construction approach were introduced. This method with the characteristics of “preserving moisture and antiseepage” and “softness and hardness” could resolve this kind of expansive rock slope problem effectively and economically.