This article briefly describes variou hypotheses of the genesis of Cenozoic basins in northwest Pacific margin and emphasizes the analysis principle of basin genesis.The basins' spatial geometric forms deduced from the hypotheses of back-arc active extension

back-arc passive tension and regional left-lateral extrusion do not coincide with the actural ones;and the deduced block movement and displacement do not balance in space.The hypothese of regional right lateral shear pulling-apart is the starting point in revealing the genesis of the basins in the northwest Pacific Ocean margin.Finally the feature of ocean crust of marginal sea basins was discussed with Japan Sea and South China Sea as examples.It is suggested that the so-called ocean crust is acturally the thinned continental crust.