Abstract: The paper demonstrates the mechanical model of wellbore stability,analyzes and estimates the equalized pressure equivalent density of the formation in Datianchi structure,in which there is a group of flowing wells,based on drilling and logging data of the area.The paper presents a design method of the equalized pressure equivalent density,through analyzing the influence of dip and soak time on wellbore stability.It will provide a theoretical basis and design method for drilllng fluid density,due to its reliab1e and simple for field use.