Abstract: The bedrocks for a proposed large power station contain a set of river and lake facies sedimentary rocks. The rocks contain conglomerate,sandstone,siltstone and mudstone and have a variety of lithologies. Different rocks have different mass deformation parameters. Conventional deformation test can only get a single lithology of rock mass deformation modulus. The soft and hard interlayered rock mass has an integrated deformation modulus that is a function of the single rock mass deformation moduli,the rock bidding angles,and the rock layer thicknesss. How to assess the integrated deformation modulus is an important task. At first,the paper conducts a statistic analysis on the proportion of different lithologies at dam site and obtains that the thickness ratio of the sandstone,siltstone and mudstone are 6：2.5：1.5. According to the statistics,the test area containing sandstone,siltstone and mudstone along the dam axial is selected. The bearing plate size is defined to be 200 cm in length and 37 cm in width for the insitu test. At the same time,the bearing plate can cover the sandstone,siltstone and mudstone. The plate can transfer the maximum applied load of 250 tons onto the underlain multilayered rocks. As a result,the integrated deformation of the bedrock can be determined. The experimental results show that for the weak weathered bedrock at the rock dam foundation,its integrated deformation modulus can be up to 7.0 GPa. This value meets the design requirements. 