The broken rice starch was prepared from the rice processing by-products. By single factor tests and orthogonal experiments, taking the extracting rate and purity as the indexes, the rice starch was prepared by alkaline hydrolysis method of microwave-assisted with alkaline protease extraction. The optimum conditions were as followed: the ultrasonic time 25 min, enzyme content 5 mg/g, enzyme hydrolysis time 2 h, enzyme hydrolysis temperature 45 ℃, ratio of solid to liquid 14 (g/mL). Under the conditions, starch extraction yield reached 98.56%, and the purity was 99.13%.