Nickel is an indispensable resource in national defense, aerospace, energy, transportation, petrochemical and other fields, and is also one of the most important strategic non-ferrous metals in China. As nickel demand increases, nickel sulfide ore resources are rapidly decreasing and tending to be depleted. Nickel oxide ore offers new possibilities for future nickel smelting due to its abundant resources, low mining costs and other strengths. In order to better utilize chromium-bearing nickel oxide ores, it is necessary to conduct research on the extraction and separation of valuable components from such ores. The effects of liquid-solid ratio, reaction temperature, reaction time, and stirring rate on the dissolution rate of nickel, iron, and magnesium were investigated to determine the appropriate water dissolution process conditions. The kinetics of the dissolution of nickel, iron, and magnesium in the roasting material of nickel-chromium mixed oxide ore roasted with ammonium sulfate was investigated. The nickel-chromium mixed oxidized ore was roasted with ammonium sulfate under the conditions of ammonium ore ratio of 1.7:1, roasting temperature of 748 K, and roasting time of 2 h. The effects of various factors on the dissolution efficiency of nickel, iron, and magnesium in roasted materials were investigated. The suitable dissolution conditions were as follows: reaction temperature of 353 K, reaction time of 60 min, a liquid-solid mass ratio of 5:1, and stirring rate of 500 r/min. The dissolution efficiencies of nickel, iron, and magnesium reached 94.3%, 88.7%, and 84.9%, respectively. The dissolution of nickel, iron, and magnesium from roasted materials can be divided into two stages: 0~15 min and 15~40 min, which were both controlled by external diffusion with no solid product layer formation.