Abstract: To study the occurrence and developing law of eroded valley on the side slope of expressway,2.5 km of Guyuan interchange area,segment from Tongxin to Guyuan of Yinwu Expressway was selected as the experiment site.According to different slope types and preventive measures,six field experimental plots were set up in the rainy season of 2006.The occurrence and developing process of the eroded valley was investigated by volume method.The rainfall amount was measured by siphon recording rain gauge.The results showed that the amount of gully erosion cannot be kept with the rainfall amount efficiently.But it was relative to the average intensity of rainfall in a certain extent.When the average intensity of rainfall exceeded 9.47 mm/h,gully erosion was likely to happen.And the bigger the average intensity of rainfall is,the more the gully erosion is produced.Under certain average intensity of rainfall,the more the rainfall amount is,the more the gully erosion is.The measure of hexagon brick can prevent the eroded valley from happening.The preventive measures of arch beam can control the development of eroded valley.Combining them with vegetation measures,the slopes can be defended better.The eroded valley on the slopes of dump area developed acutely,where the amount of gully erosion can reach 9 779.55 t/km² during a rainy season.