The invention discloses preparation and application of a photocatalyst Ag/ AgBr, and belongs to the technical field of nanocatalysts. According to the method, 1-butyl-3-methylimidazole bromine ([BMIM]Br) is prepared from N-methylimidazole and n-butyl bromide under certain conditions, wherein a preparation method comprises the steps that a certain amount of N-methylimidazole and a certain amount of n-butyl bromide are weighed and put into a round-bottom flask to be stirred and refluxed for a period of time in a high-temperature water bath to obtain faint yellow thick liquid, and 1-butyl-3-methylimidazole bromine ([BMIM]Br) which is collected after cooling has the very high catalytic activity in preparation of the photocatalyst Ag-AgBr by serving as a precursor.