Abstract: The stability of crown surrounding rock masses of great span underground caverns is important to successfully and safely construct. The crown blocks going against stability of surrounding is prevalent because that the surrounding rock masses of Three Gorge underground powerhouse is heavy jointed and very hard. Adopting working process of information construction geology, the crown blocks incised by uncovering structure plane are fleetly searched and located and their stabilities analyzed by the program (GeneralBlock) of 3D-blocks, it is the guidance to shorting and construction of crown. The investigations to crown blocks show that: 43 movable blocks are revealed, hereinto, 11 blocks is instable and 16 blocks is potentially instable, the measures of reinforcing are implemented in time. The flow and method of the investigation can be consulted by analogous projects.