The invention relates to a method for determining cross arm leeside load decreasing coefficient of a transmission tower. The method is determined based on a wind tunnel experiment. The method comprises the following steps: manufacturing a wind tunnel model; separating cross arm wind load; determining a cross arm shape coefficient; determining a cross arm windward side shape coefficient; determining a cross arm leeside shape coefficient; and determining the cross arm leeside load decreasing coefficient. According to the method, the more accurate cross arm leeside load decreasing coefficient based on the wind tunnel experiment can be provided.