In the process of wheat refining, 23% ~ 25% wheat bran, 3% ~ 5% flour head and 0.7% ~ 1.5% germ can be obtained, and the main product is edible flour. The production process of wheat flour, wheat bran and secondary flour is amorphous, and the products of all grades are mostly linked according to the quality specifications of products or by-products. In the process of wheat milling, different grades of products or by-products such as refined flour, standard flour, secondary flour and wheat bran can be produced. Peel contains a lot of crude fiber, which is difficult to digest and absorb, while embryo is rich in fat, which affects the color and baking quality of wheat flour. High ash content in aleurone layer will increase the ash content of wheat flour and lower the grade. The traditional principle of wheat processing is to fully extract the endosperm part of wheat grain and separate husk, embryo and aleurone layer as much as possible. In order to achieve this goal, the gradual grinding method is generally adopted, that is, once grinding and then screening is called a system. The first step is to peel the wheat grains to obtain bran flakes (with endosperm) and relatively pure endosperm grains and flour, commonly known as one-skin grinding. After the bran flakes with endosperm enter the two-stage leather grinding system, finer endosperm particles and flour with particle size not up to the finished product specifications are obtained, and then enter the next central grinding system for further processing, which is repeated in turn. According to the length of the working procedure and the requirements for the quality of the finished wheat flour, there can be 3-9 or more passes. A certain proportion of flour can be obtained by each skin mill or heart mill system. According to the requirements of finished products, all the flours obtained from each system can be mixed into one kind of wheat flour, or the flours from several systems can be combined separately to form different grades of wheat flour. Poor quality flour is difficult to sell as secondary flour. At present, grading flour substitution technology is mostly used, while the traditional two-stage flour substitution technology is mostly used in small flour mills or mills.