The invention aims to provide an extrusion processing method of nutritious breakfast cereals. In the method, mixed grain rice such as millet, oat, soybean, corn and black bean is utilized as raw materials, a formula is designed on the basis of a nutrient balance theory, the nutritious breakfast cereals are produced by adopting a modern twin-screw continuous extrusion cooking technology and deep development is performed on the mixed grain rice, thus realizing the purposes of 'coarse grain, eat fine' and 'combination of coarse and refined grains', improving the values of agricultural products such as the mixed grain rice and achieving good economic benefit and broad market prospect.