The process of shock waves propagation in the target material is analyzed theoretically when hitted by a flier-plate with graded wave impedance. Under the impact, the compression curve of target consists of small Hugoniot curves originating from different original states, which are located between the Hugoniot and the isentropic curves. Thus any state between Hugoniot and isentropic curves can be obtained by controlling the number of flier-plate's layers. It provides a theoretical basis for experimental study of off-Hugoniot equation of state. The validity and reliability of the theoretical analysis are supported by the consistency of experimentally measured and calculated interface velocity values.