A solution culture experiment was conducted to determine the effects of co-application of Mo and Se on valence of Se and chemical forms of Mo and Se. The results showed that application of Se increased alcohol soluble and water soluble Mo concentrations in roots and shoots and decreased salt soluble and acetic acid Mo concentrations in roots and shoots of Chinese cabbage. Application of Se had no significant effect on shoot hydrochloric acid Mo concentration, but increased root hydrochloric acid Mo concentration. Application of Mo had no significant effects on shoot alcohol soluble and water soluble Se concentrations, but increased salt soluble and hydrochloric acid Se concentrations and decreased acetate acid Se concentration. Application of Mo had no significant effect on root alcohol soluble Se concentration, but decreased water soluble Se concentration and increased salt soluble, acetate acid and hydrochloric acid Se concentrations. Application of tetravalent Se was conducive to the conversion of inorganic Se to organic Se and application of Mo can promote inorganic Se transform to organic Se in high Se level. Therefore, co-application of Mo and Se had antagonism and synergies on Mo and Se chemical forms, but conducive to the production of organic Se-rich crops.