The invention relates to a method for breaking a tail of a cow. The method comprises the following steps of step S1, performing tying with a rubber band at a position of the 6th to 8th tailbone from the upper end of the tailbone of the cow; and step S2, after one month, enabling the cow tail at the lower end of the rubber band to automatically drop. Through the adoption of the method for breakinga tail of a cow provided by the invention, the cow tail can effortlessly and conveniently drop off, besides, discomfort of the cow cannot be caused, and milk yield and the quality of dairy products cannot be influenced; and besides, the dropping length of the tail is so proper that the situation that the cow tail is in contact with the ground when the cow squats and lies can be avoided, and hygiene of a milk source cannot be influenced due to swaying of the cow tail when the cow is milked.