The invention relates to a chemical-biological passivator and a preparation method thereof. The preparation method comprises the following steps: uniformly mixing ferrihydrite with carbon residue powder to obtain a mixture; adding the mixture into a bacterial suspension of acanthosporum, the bacterial content of which is 1.0-9.9*10<7>cfu/ml. The chemical-biological passivator provided by the invention is obviously enhanced in promoting growth of water spinach, can reduce arsenic absorption by water spinach and reduce effectiveness of soil arsenic, and can be used as an efficient chemical-biological passivator for regulating arsenic pollution farmland risk in the future.