A catalyst in a calcined state has a specific surface area of 20-50 m2/g of catalyst, and a specific surface area of nickel metal after reduction of the catalyst of 8 to 11 m2/g, wherein the average particle size of nickel metal is 3-8 nm, the dispersion of the particles is 10-16%, and the content of nickel is 5-15 wt. % based on the weight of calcined catalyst. A support has a specific surface area of 40-120 m2/g with a pore volume of the support of 0.2-0.4 cm3/g, wherein the support is selected from a mixture of zirconium oxide and cerium oxide or magnesium oxide, cerium oxide and the ballast being zirconium oxide. The catalyst further contains a promoter selected from the group consisting of palladium and ruthenium, in an amount of from 0.01 to 0.5 wt. %. The catalyst is prepared by co-precipitation with ammonium hydroxide from a solution containing nickel, cerium and zirconium precursors and distilled water or from a solution containing nickel, cerium, zirconium, and magnesium precursors and distilled water, and having a pH of 8.0-9.0. The process is carried out under agitation at a temperature of 40-45° C. for 1-2 hours, followed by filtration, drying at a temperature of 100-110° C. for 6-8 hours, and calcining at a temperature of 400-650° C. for 4-6 hours. The invention provides a high average conversion of natural/associated gas of at least 90% in an autothermal reforming reaction of natural or associated gas, and a high synthesis gas output of at least 7000 m3/m3cat·h.