Abstract: This paper analyzed the characteristics of horizontal wells damage,and then established a set of method to calculate the drilling fluid lost circulation and pollution depth that vary with the horizontal section.Based on the lost circulation characteristics of drilling fluid and law of conservation of mass,by way of Cart model that applies in the fracture fluid lost,the paper came to a mathematics model of drilling fluid lose that varies with the horizontal section.Applying the theoretical model to calculate the equifinal damage depth which varies with the horizontal section,a formula of pollution depth which varies with the horizontal section through volume transformation was presented.