The invention provides a method for extracting natural active substance from non-heading Chinese cabbages and application thereof. The natural active substance is prepared through the following steps that the non-heading Chinese cabbages undergo vacuum package by using polyethylene films and are rapidly frozen with liquid nitrogen after being harvested, smashing is performed after vacuum freeze drying, smashed cabbages are subjected to extraction by using a 65-75% ethanol water solution under the ultrasonic condition, extraction liquid is concentrated to be almost dry, and a crude Chinese cabbage extract is obtained; the crude Chinese cabbage extract undergoes ultrasonic dissolving with distilled water to obtain water suspension, extraction is performed by using ethyl acetate, and the extraction liquid is evaporated and concentrated and then is subjected to freeze drying to obtain the natural active substance, wherein the main components include flavonoids, polyphenols and other bioactive substances. The target product prepared by means of the method has stronger biological activity and can remarkably improve the immune level of cells, enhance body immune functions, remarkably improve the meat production capacity, immune functions and antioxidant level of raised animals and improve the expression level of cell factors mRNA after being applied.