Abstract: The diversion tunnel of Jinping-Ⅱ Hydropower Station is an oversized underground project. It is noted for its deep location, long length and large diameter. The common depth of the diversion tunnel is 1 500 to 2 000 m, with the maximum depth of around 2 525 m. In view of the presence of ultrahigh geostress, which may exceed 70 MPa, violent rock burst and deformation usually occurred during the tunnel construction. In addition, core discings were usually found in boreholes, which made it difficult to measure the geostress through conventional aperture-strain method and hydro-fracturing method. To measure the geostress in deep rock mass, we developed a measurement system for the ultrahigh pressure hydro-fracturing geostress. This system consists of specifically designated high-pressure oil pump, seamless steel tube, packer, oil tube, and measurement instruments. It has been used at Jinping-Ⅱ Hydropower Station and good results were achieved. This system provides with an insight for the measurement of ultrahigh pressure geostress in fields of highway, railway, hydropower, and mining.

Key words: Jinping-Ⅱhydropower station, ultrahigh pressure, geostress, hydro-fracturing, measurement system