Abstract: The geological conditions of this underground hydroelectric power plant is very com- plex. The high angle faults, low angle faults and fractures in the same directions have developed under the influence of areal geological structures. The stability and perfection of the rock pillar between the main machine hall and the tailwater surge chamber was spoiled by longitudinal and transversal fractures which intersected each other and dissected the rock pillar into more than ten rock blocks. Simultaneously, some rather instable rock masses were formed in some parts of the upstream and downstream side-walls of the main machine hall. Besides, in consequence of the insufficiency of geological data and rock test results, the choice of the axis of the main machine hall and the layout of underground structures are not quite ideal. The density of rock chambers is very big, and the intersections of the chambers with lateral galleries totalled nineteen, thus enhanced the difficulty of construction of the underground power plant, Therefore, some suitable engineering structural measures were adopted at the principal positions, such as: the use of deep rock anchor piles, hollow anchor rock chambers, lateral buttress and propped beams; rectification of local layout; the adoption of a more reasonable construction schedule, and a series of constructional safety measures, such as, capping, locking, anchoring, thrusting, bracing, hooping etc. From prototype observations and the results of finite element analysis of the stress and deformation calculation of the surrounding rock1, it is evident that these measures are effective and ensured the completion and safety operation of underground power plant. Further prototype observations and investigations are still under way.