A process for producing ammonia synthesis gas from the reforming of hydrocarbons with steam in a primary reformer (1) equipped with a plurality of externally heated catalytic tubes and then together with air in a secondary reformer (2) is characterized in that the reaction of said hydrocarbons with said steam in said primary reformer (1) is performed at an operating pressure of more than 35 bar in the catalytic tubes, in that air is added to said secondary reformer in excess over the nitrogen amount required for ammonia synthesis and in that the excess of nitrogen is removed downstream the secondary reformer preferably by cryogenic separation or by molecular sieves of the TAS or PSA type. This process allows to obtain high synthesis gas production capacities and lower investment and energy costs.