In order to research the stability of high and steep composite medium slop consisting of loess and coal wastes layer by layer,on the basis of analyzing the factors affecting the stability,the mechanics model for the roots of plant reinforcing surface soil of slope was built.The contribution of the roots of plant to the improvement of mechanical property of soil was deduced.A numerical simulation was made on the stability of slope of some debris retaining dam using FLAC software.It was pointed out that the slope will slide under certain condition.It was suggested that the measure of surface layer reinforcing combined with deep layer reinforcing should be took to the slope.The result show that the measure is practical and effective,can ensure the stability of the slope.It provides practical experience for the improvement of mechanics surroundings of similar slope and its effective treatment.