The method provides a method for preparing chromium hemitrioxide by red-soil nickel ore; roasting materials including the red-soil nickel ore and alkali carbonate which carry out molten salt reaction carry out liquid-solid separation and aluminum removal by carbon division to obtain alkali immersion liquid which is rich in chromate and alkali carbonate, the alkali immersion liquid is used as raw material, carbon dioxide is used as acidulant, and the alkali immersion liquid and the carbon dioxide and organic matters such as alcohol and aldehyde and the like carry out liquid phase reduction reaction directly to generate hydration alkali calcium carbonate chromium salt slurry under the conditions with a certain temperature and pressure, the slurry carries out liquid-solid separation to obtain hydration alkali calcium carbonate chromium salt filter cakes and filtrate containing less hexavalent chromium and carbonate; the filter cakes are dried, calcined at high temperature and washed to obtain the final product chromium hemitrioxide, and the filtrate is returned to burdening working procedure to realize circulating burdening. The method can realize comprehensive utilization of valuable metal chromium in the red-soil nickel ore, the diversification of the red-soil nickel ore product is realized, and the method is in accordance to requirements of cleaner production, so as to realize internal circulation of material and reaction mediums; in addition, the reduction process flows are simple and the industrial operability is strong.