[en] By reviewing the property of the radiative shock Rankine-Huguoniot relations for different values of the net radiative flux to the pre-shock and plotting the Rayleigh and Hugoniot relations at the plane, we find that the branches of the hyperbolic (Hugoniot locus) will present an abrupt change that its two top-right and down-left branches will change to down-right and top-left ones if the net radiative flux to the pre-shock larger than a certain value, and we define t such a value as the threshold of the net radiative flux to the pre-shock. The change will result in a compression ratio larger than for ideal gases with index. We also demonstrate the effect of the radiative flux on the compression ratio with radiation hydrodynamics code RDMG. (authors)