The invention relates to the technical field of chemical industry, and discloses a method for preparing sodium dihydrogen phosphate and preparing a special fertilizer for Rosa roxbunghii as a byproduct. The method comprises the following steps: reacting phosphoric acid with carbomite by analyzing nutritional elements required by Rosa roxbunghii to prepare an intermediate, reacting the intermediate with an alkaline sodium salt to prepare sodium dihydrogen phosphate, controlling the content ratio of nutrients in a filtrate in the sodium dihydrogen phosphate crystallizing process, and adding potassium and other trace element compounds to the sodium dihydrogen phosphate separated filtrate in order to prepare the special fertilizer for Rosa roxbunghii. The prepared special fertilizer for Rosa roxbunghii can improves the output of Rosa roxbunghii, and the prepared sodium dihydrogen phosphate has a high purity and a relatively low cost.