Abstract: Through the analysis of characteristics in curve of deformation tests on the four kinds of rockmasses such as claystone, siltstone, feldspar-quartz sandstone and lithic sandstone underlain the dam foundation of Tingzikou Hydropower Station, it is thought that: the test deformation of thick lithic sandstone is mainly caused by slippage deformation of rock crystal lattice ； the test deformation of feldspar-quartz sandstone is mainly from small crevices in rock, compressive deformation of rock small holes and slippage deformation of rock crystal lattice; and the test deformation of thin layer claystone in which horizontal layer is developed and siltstone is mainly from compression deformation in interlayer, small crevices in rock, compression deformation of rock small holes and slippage deformation of rock crystal lattice.

Key words: Tingzikou Power Station , layer rockmass , deformation test , curve characteristic