It′s very important for the directional well drilling stability and security, to know the changes of the formation collapse pressure with the well deviation and azimuth angle under different formation stress conditions. Here we analyze the relationship among equivalent mud density of formation collapse pressure, well deviation and azimuth angle, by numerical simulation of formation stress for four cases. And the changes of formation collapse pressure in the directional wells are also discussed. The result will provide a reference to determine the security mud density in directional well drilling.