Abstract：The key block has received a lot of attention in underground engineering because it loses stability first after excavation. The block theory originally proposed by Shi doesn¢t deeply discuss the geometric characteristics and explain how to determine the precise locations of the key block. Therefore，the Unwedge program based on key block theory can not predict complicated polyhedral block and determine its precise location. In this paper，the stability of key block of a large underground excavation is analyzed in detail. First，the Unwedge program (Ver.2.35) is used to identify possible key blocks and some limitations are discussed. Then，many kinds of geometric models of polyhedral key block are set up and a new computer program is developed to search key blocks and determine their precise locations considering the action of gravity and dynamic loadings. Finally，the results of the two methods are compared and the differences between them are analyzed.
