Inter-visibility calculation is to calculate the visible area sets of known observing points.The common methods,including R3,PRVA,PREAI,R2 and Xdraw,are complex and not adaptable for military application.In this paper,the advantages and disadvantages of each algorithm are analyzed comprehensively.The Xdraw method is selected and improved for increasing the calculation efficiency.On the basis of Digital Elevation Model (DEM),simulation is made under VC environment.The result shows that the improved Xdraw algorithm can not only calculate the inter-visibility situation of surface terrain,but also compute the inter-visibility of space of any height layer precisely and quickly.Therefore,its feasibility and effectiveness for threat inter-visibility calculation are proved.