The deformation characteristics of rock mass and the coefficient of subgrade reaction are directly related to the internal force of foundation, the final deformation and the uniformity of deformation. Reasonable determination of the coefficient of subgrade reaction is important for high-rise buildings, especially super high-rise buildings. Based on the demonstration of a 500m super-high-rise building in Chengdu, the plate loading tests with different sizes and shapes of pressure plates were carried out in the underground flat tunnel for soft clayey rocks in the bearing layer of the building, and the UCS, conventional triaxial compression test and direct shear test of rock blocks were carried out, the relationship between UCS, elastic modulus, bearing plate size and coefficient of subgrade reaction used to analyze based on laboratory and in-situ test data. The results show that the coefficient of subgrade reaction has a tendency to increase with the increase of uniaxial compressive strength of the rock, and the smaller bearing plate test data is more discrete, the larger bearing plate test data is less discrete; if the coefficient of subgrade reaction of red-mudstone layer is revised according to the empirical formula of clay or sand recommended by the code, it will cause great error, and the coefficient of foundation bed of medium weathered mudstone has a hyperbolic empirical relationship with the size of load plate, based on this, it is suggested that the coefficient of subgrade reaction of red mudstone of building should be modified, for the coefficient of subgrade reaction of red mudstone obtained by 500mm square bearing plate test is less discrete.