Abstract: The first, most important and most prominent one among Mr Gu Dezhen's academic thoughts, attaching great importance to practice, geomechanics, mechanics of rock masses and research of stability of rock masses is the thought, attaching great importance to practice. The thought is, in a nutshell, attaching great importance to geological inspect and investigation in earlier stage of a project and taking an active part in the design and in the construction. This paper firstly sets forth the process of forming the thought, attaching great importance to practice, and its specific reflection; then with a case history of a project, investigation o foundation rock and design and construction of foundation structure of the first double-use great bridge after liberation of eight piers, nine holes and steel framework, spanning water level 1200m wide of Changjiang river, reflects specificly and lively the thought; and finally points out Changjiang three gorges hydropower project is the one Mr Guo paid a good deal of attention to and made much of effort in during his life-time, and the thought has played a important guidance role in geological investigation, design and construction of the second-stage projects of comprehensive control and prevention of geological hazards in Changjiang three gorges reservoir and will play still the role in the three-stage projects.