Flier-plates with wave impedance gradient can be regarded as a new kind of FGM, which has come to show great potential for the application in dynamic high-pressure technology. In the present paper, theoretical model of generating quasi-isentropic compression via W-Mo-Ti system FGMs has been established, and study on the numerical simulation was carried out. The results show that, the process of quasi-isentropic compression on target can be seen as the successive overlap of a series of small shock loading waves generated by the transient layers. The design for flier-plates with graded wave impedance should conform to the parabolic or cubic function relationship between the impedance and the thickness. And the thickness of target should also be well designed as for creating a better quasi-isentropic compressive wave profile.