The issue of inter-visibility indicates to the problem of the set of terrain surface area observable to known viewpoints and computed viewpoints.The common methods are R2,R3,PRVA,PREAI and Xdraw.But in the real application,especially when the observed target is an aircraft,what should be known includes the inter-visibility situation of surface terrain and of space of any flight levels.This paper proposes an improved inter-visibility algorithm based on minimum inter-visibility height.Based on the Digital Elevation Model (DEM),this method could not only calculate precisely the inter-visibility situation of surface terrain,but also compute the inter-visibility of space of any flight levels precisely and quickly.Then the algorithm was implemented by programming,which identifies the feasibility and accuracy of the algorithm.