Disclosed are a catalyst for preparing synthesis gas from natural gas and carbon dioxide, and a method for preparing the same. More particularly, a combined reforming process is performed as an economical way of using carbon dioxide, wherein steam reforming of natural gas is carried out simultaneously with carbon dioxide reforming of methane in such a manner that a predetermined ratio of carbon monoxide/carbon dioxide/hydrogen (H2/(2CO+3CO2)=0.85-1.15) is maintained. In this manner, the catalyst is used to prepare synthesis gas suitable for methanol synthesis and Fischer-Tropsch synthesis. Disclosed also is a method for preparing synthesis gas on a specific catalyst consisting of Ni/Ce/MgAlOx or Ni/Ce-Zr/MgAlOx. The catalyst is inhibited from deactivation caused by generation of cokes during the reaction as well as deactivation caused by reoxidation of nickel with water added during the reaction. Therefore, the catalyst shows excellent activity as compared to other catalysts for use in combined reforming.