Disclosed in the present application are a method for optimizing an O/A ratio in ion extraction of nickel-cobalt (manganese) from laterite nickel ore and use thereof. The optimization method comprises the following steps: obtaining a liquid parameter of a liquid to be extracted, a parameter of an extractant and a target extraction efficiency of manganese; constructing a theoretical O/A ratio model and determining a theoretical O/A ratio based on the liquid parameter, the parameter of the extractant, the target extraction efficiency of manganese and the theoretical O/A ratio model; and constructing a limiting condition and determining an optimal O/A ratio based on the theoretical O/A ratio and the limiting condition. In the present application, based on the combination of theory with production practice, aiming at the selection of the O/A ratio of the extraction section in the nickel-cobalt (manganese) co-extraction process by different extractants and the solution to avoid the generation of the third phase in the extraction, a corresponding theoretical model is established, and the optimal O/A ratio is determined on the basis of the theoretical model, and ultimately, the nickel-cobalt (manganese) mixed solution obtained from co-extraction with the optimal O/A ratio is to be used in the preparation of nickel-cobalt (manganese) precursors and ternary cathode materials.