The key block theory is widely used in stability evaluation of engineering fractured rock mass. The searching method of key block should be of pertinence and flexibility due to large amount of joints and huge calculations caused by the searching of very few occasional blocks. Firstly

the researching region is decomposed into convex sub-regions; and all closed loops on free surfaces of research region are found out. Then the primary joints that form the closed loop and relevant joints that intersect with the primary ones are regarded as infinite plane to cut the region into convex blocks

among which the key block associated to closed loop will be identified. With better pertinence and programmability

this method can fit some artificial conditions very well such as wedge searching and limitation of trailing cutting. It can also search concave block successfully. Finally

an example of top pillar is presented to show the effectiveness of the above-mentioned searching method.