The invention discloses a method for preparing a chrome tanning agent from a waste chrome tanning liquid. The method is characterized by comprising the following steps: collecting the waste chrome tanning liquid after tanning is completed, precipitating chrome by using alkali, and performing plate-frame pressure filtration to obtain chrome cakes; performing acidolysis on the chrome cakes by using sulfuric acid, cooling, filtering to obtain filtrate, adjusting the pH value to 2.0-2.5, separating by using a cation-exchange chromatography, measuring the content of the component chrome, adding corresponding parts of sodium formate and sodium acetate according to the contents of negative and neutral components, dissolving, boiling to concentrate until the final solid content is up to 50%, cooling for one hour, separating by using the cation-exchange chromatography, measuring the content of the component chrome, performing aging reaction for 4-7 days according to the contents of the negative and neutral components until the contents of the negative and neutral components are up to 75-84%, diluting for 6 times with water, adjusting the pH value to be 3.8-4.0, placing to stand for 6 hours until the contents of the negative and neutral components are 40-50%, and the content of a high electric charge component with the valence greater than +4 is 17-22%. The method for preparing the chrome tanning agent from the waste chrome tanning liquid is simple and flexible to operate, high in yield, excellent in property of the tanning agent, and applicable to tanning of first-layer high-class leather and second-layer leather of cattle, sheep, pigs and the like.