Abstract：The model tests for underground houses，in the past，were mostly two dimensional or three dimensional in small size. In those model tests，the excavation process was not simulated in its real meaning，or even a ready-made tunnel or a cavern was simply put into the model before loading，which is unreasonable because the tunnel should be excavated under the condition of initial stress field. Many researchers improved the situation by loading the model prior the excavation simulation. However，the excavation was modeled by pulling some rigid blocks or withdrawing inner pressure applied by hydraulic jack from the ready-made tunnel or cavern. But this can only be done for the plane model test，and was surely not the best way to simulate the excavation process. Recent years，more and more large，even huge underground projects with cavern group are under planning，designing and construction in China，as well as in the world. Besides the numerical simulation，physical and geo-mechanical model tests have been putting on the agenda to meet engineering request. The technique of concealed excavation in a three dimensional geo-mechanical model test for an underground powerhouses of a large hydropower station is studied in present paper，including the determination of basic principles of concealed excavation，the design of positioning systems inside and outside of the model，special designed micro-step-forword boring machine and devices，internal monitoring system by using the mini-infrared-camera and introduction to the process of concealed excavation.
