The invention relates to the technical field of preparation of diamine and polyamine of a diphenylmethane series, in particular to a composite catalyst for catalyzing the decomposition of an amine salt as well as a preparation method of the composite catalyst and a method for preparing MDA. The composite catalyst is prepared from a carrier, primary active components and secondary active components. The method for preparing the diamine and polyamine of the diphenylmethane series comprises the following steps: (a) enabling phenylamine to carry out salt forming reaction in the presence of an acidic catalyst to generate phenylamine acid salt; then enabling the phenylamine acid salt to contact a formaldehyde solution to perform condensation reaction; and (b) in the presence of the composite catalyst, performing the catalytic decomposition on a condensed product to obtain a gaseous product containing HCl and a liquid product I containing the diamine and polyamine of the diphenylmethane series. By adopting the method provided by the invention, the quality of the MDA produced subsequently can be fundamentally improved, and the cost is also saved.