Abstract: Based on modal perturbation method, a new approach is developed for solving the random seismic response of horizontal layered soil. In the approach, the dynamic characteristics of layered soil are calculated approximately in the modal subspace spanned by several lower models of equivalent homogeneous shear beam with the same depth, average shear modulus and mass density. As a result, the differential vibration equation of soil layer with variable properties is transferred as a set of algebraic equations. Then the equation of random seismic response of layered soil is derived. By way of numerical example, the procedure and calculation error of random seismic response by using mode perturbation is discussed. Numerical results show that the approach is simple and practicable, that error is less than 5%.