The invention relates to a method for preparing arsenazo II. The method comprises the steps: (1) carrying out preparation: subjecting aminophenylarsonic acid, diluted hydrochloric acid, a sodium nitrite solution and an aqueous solution of chromotropic acid disodium to a reaction so as to produce a crude arsenazo II product, wherein the mole ratio of aminophenylarsonic acid to chromotropic acid disodium is 1: (0.5 to 0.6); (2) carrying out acidification: filtering out solids in the reaction solution in last step, dissolving the solids into a certain volume of dilute hydrochloric acid under warm conditions, and carrying out aging overnight so as to precipitate solids; and (3) carrying out drying: filtering out solids in the reaction solution in last step, soaking the solids into a certain volume of dilute hydrochloric acid under warm conditions, filtering out solids again, and carrying out drying, thereby obtaining the finished arsenazo II product. According to the method disclosed by the invention, expensive special equipment is not required to be employed during production, the production cost can be reduced, and the market competitiveness of enterprises is improved, so that the product can be more extensively popularized and applied on the market.