The invention relates to a compound mixed slow release fertilizer using perlite as a carrier, which is characterized in that perlite is used as the carrier, 0-5-10% sulfuric acid and a 20-40% nitrogen fertilizer (a urea water solution) are mixed in a quantity-equal manner and then mixed with a 10-30% potassium fertilizer (such as a potassium sulfate water solution) in a quantity-equal manner or a quantity-unequal manner, and the weight ratio of raw materials is as follows: 3 parts of water solution is compounded with 1 part of expanded perlite and 2 parts of phosphorus-containing powdered fertilizer. The preparation method of the compound mixed slow release fertilizer comprises the following steps: mixing the sulfuric acid, the nitrogen fertilizer and the potassium fertilizer water solution; spraying onto perlite granules; spraying the phosphorus-containing powdered fertilizer while spraying the mixed solution in a granulator to form a granular three-element compound mixed slow release fertilizer; and drying to obtain the finished product. Through the high adsorptivity of the perlite, the three main elements necessary for plants can be released slowly, and the fertilizer can be prevented from being quickly washed away by rain water; and meanwhile, the hardening of soil can be improved, the loose permeability and the moisture and fertility preserving capability of soil can be increased, and no three wastes are produced.