The invention discloses a tetravalent platinum complex oxidant and a preparation method and application thereof in synthesizing of disulfide bonds in a polypeptide molecule. The tetravalent platinum complex oxidant has a structure shown in the formula (I), wherein n is 0 or 2; X is halide ion with -1 valence. The preparation method of the tetravalent platinum complex oxidant comprises the following steps of enabling potassium chloroplatinite and ethylenediamine dihydrochloride to react, so as to obtain dichloro(ethylenediamine)platinum; enabling the dichloro(ethylenediamine)platinum and a heteroaromatic ring containing two nitrogen atoms to react, so as to obtain an ion type bivalent platinum complex; enabling the ion type bivalent platinum complex and a hydrochloric acid solution containing hydrogen peroxide to oxidize, so as to obtain the tetravalent platinum complex oxidant. The tetravalent platinum complex oxidant has the advantages that the residual radicals of methionine are not oxidized, the reaction yield rate is high, the reaction rate in the stronger acid reaction medium is quick, and the cost for synthesizing the disulfide bonds in a pharmaceutical molecule by using the oxidant is greatly reduced.