The E10.7 index representing the solar EUV irradiance flux has been used to replace the traditional F10.7 index in more and more researches and applications. X-rays are important ionization source of the terrestrial D and E layer. Due to lack of enough observation to the D layer and versatile ionization sources of E layer, it is difficult to study the influence of X-rays to terrestrial ionosphere. The dominant ionization source for the secondary layer of Martian ionosphere is soft X-ray. This makes it possible to study the X-ray influence on the ionosphere. By studying the dependence of Martian ionospheric secondary layer peak density to E10.7, it is found that even after necessary corrections, the dependences are in disagreement for different data sets. After data and theoretical analysis, a new index Xs is introduced to represents the solar soft X-ray irradiance flux. After replacing E10.7 with this Xs index, the peak density dependences upon the solar irradiance are in good agreement for different data sets, indicating Xs index is a better measure of solar soft X-ray irradiance than E10.7 index.