As there is a great controversy about the buoyancy reduction coefficient of saturated silty clay foundation, I adopt a buoyancy measurement system, with complete seepage path, that can eliminate side wall friction and base negative pressure and can accurately judge floating moment of the inner box. The total pressure at the bottom of the box and the pore water pressure in the silty clay is tested to get its reduction factor of 0.744. Reduction coefficient for silty clay of 0.754 is obtained by the numerical simulation software COMSOL Multiphysics, which is not much difference from experimental results. The ternary linear regression equation of the reduction coefficient of silty clay is established by linear regression method for collinearity diagnostics of independent variables in equations, excluding the dry density and porosity of the independent variables. Finally, it is determined that the reduction coefficient of red sandstone and silty clay is only related to permeability coefficient.