The invention discloses a preparation method of diphenyl phosphite. The method comprises the following steps: taking phosphorus trichloride, phenol and water as raw materials; after reacting, decompressing and introducing nitrogen gas to remove a byproduct hydrogen chloride, so as to obtain the diphenyl phosphite. According to the method disclosed by the invention, the byproduct phenol is not generated and the problems of cost increasing and waste liquid treatment, caused by repeated recycling of the phenol are solved; meanwhile, a production link is simplified and the production efficiency isimproved; a lot of phosphorous acid does not need to be used in a production process and production risks are greatly reduced; the production cost and the environment cost are reduced and the preparation method is applicable to industrial production.