The invention relates to an azothoate III preparation method, which comprises: (1) preparing: carrying out a reaction on aminophenyl phosphate, dilute hydrochloric acid, a sodium nitrite solution and disodium chromotropate to obtain an azothoate III crude product; (2) acidification: filtering out the solid in the reaction liquid obtained in the step (1), dissolving the solid in a certain amount of dilute hydrochloric acid under a warm condition, placing overnight, and precipitating a solid; and (3) drying: filtering out the solid in the reaction liquid obtained in the step (2), soaking the solid in a certain amount of dilute hydrochloric acid under a warm condition, filtering out the solid again, and drying to obtain the azothoate III finished product. The present invention provide the production method, wherein the special equipment having the high price is not used, the production cost is reduced and the market competition ability of the enterprise is improved with the production method of the present invention, such that the product can be widely promoted and used in the market.