The invention belongs to the technical field of production and application of water-treating agents, and in particular relates to a slow-release type composite sterilization algicide. The slow-release type composite sterilization algicide comprises the following components in percentage by weight: 20 to 50 percent of bromine hydantoins, 20 to 40 percent of isocyanuric acid oxidants, 10 to 20 percent of stabilizer, 10 to 20 percent of bonding agent and 10 to 20 percent of water. The preparation method comprises the following steps of: dissolving the isocyanuric acid oxidants, the stabilizer and the bonding agent into water in proportion; mixing the mixture, adding the bromine hydantoins to the mixture in proportion, and mixing the mixture to obtain the composite sterilization algicide. Thecomposite sterilization algicide has obvious sterilization and algae removal effects on bacteria, algae and the like in a water system.