The invention provides a method for extracting cellulose nanocrystals from grass fibers by combining an alkaline process and ultrasound. The method comprises the following steps: grinding, removing pectin, removing hemicelluloses and lignin, alkali cooking, acid cooking, performing ultrasonic treatment and performing freeze drying. According to the invention, under the condition that the prepared cellulose nanocrystals remain type I crystal form, the grass fibers are creatively taken as raw materials for the first time, sequentially subjected to wax, hemicelluloses and lignin removal, alkali cooking and acid treatment and combined with high intensity ultrasound, so that the cellulose nanocrystals are prepared. According to the method, the environmental pollution brought by the traditional method for preparing the cellulose nanocrystals by using hydrolysis of high-concentration strong acids can be avoided, subsidiary agricultural products or wild herbs are taken as raw materials, the wastes are changed into valuable materials, and the method is environment-friendly and economic. The finally prepared cellulose nanocrystals have the crystallinity degree of more than or equal to 68%, the fiber a content of more than or equal to 80%, the diameter of about 3-20 nm and the length of 200-400 nm.