During the synthesis of precursor materials for ternary lithium battery cathodes via co-precipitation, the presence of impurities such as iron, aluminum, magnesium, copper, and sulfur can adversely affect the electrochemical performance of the final cathode materials. The existing studies are lack in mechanism analysis of the influence of coprecipitation conditions on the inclusion of impurities in precursor particles. The impurities in Ni0.8Co0.1Mn0.1(OH)2 synthesized via co-precipitation have been analyzed using multiple characterization methods. The impurities primarily consist of sodium, magnesium, calcium, iron, copper, zinc, aluminum, and sulfur, and may be distributed within the crystal lattice, interstitial sites, or on the surface of the precursor particles. The results indicate that the concentration of ammonia has a significant effect on the contents of iron, copper, zinc, and aluminum. The precursor synthesized under the condition of pH 11.7 is colloidal and has a larger specific surface area, which adsorbs the highest amounts of sodium and sulfur, being 28134.62 and 12898.50 μg/g, respectively. The variation in stirring speed has a certain effect on the distribution of sulfur content. Optimizing the process conditions of ternary precursor co-precipitation process is of great significance to control the impurity content in the precursor and improve the electrochemical performance and safety of the final ternary cathode material.