The invention relates to a starch-potassium iodide reagent and a preparation method of the starch-potassium iodide reagent as well as test paper, a kit and a semi-quantitative detection method of benzoyl peroxide in flour. The preparation method comprises the following steps: (1) carrying out starch treatment: carrying out degreasing oxidization treatment on starch; (2) preparing the reagent: mixing the starch obtained by the step (1) with cold water; mixing with hot water with the temperature more than 90 DEG C; heating until a mixture is clarified, and then cooling; after cooling to 45 DEG C to 55 DEG C, adding sodium hydrogen carbonate and sodium carbonate to form a buffering system; adding potassium iodide and uniformly mixing. The starch-potassium iodide test paper provided by the invention is simple and convenient to prepare, low in preservation requirements and long in guarantee period, and can be used for detecting the benzoyl peroxide in a system. The detection method of the benzoyl peroxide, provided by the invention, can realize detection of the benzoyl peroxide in the flour; the detection limit is 0.02g/L; a detection process is simple, convenient and rapid, and the benzoyl peroxide in the flour can be rapidly detected on site.