The invention discloses a method for obtaining the escape rate of ammonia in SCR denitration flue gas by using XRF, and is characterized in that the method comprises the steps: establishing a standard curve of the SO3 fluorescent intensity and the SO3 mass fraction by using the XRF; collecting smoke dust of an electric dust remover inlet flue, and calculating the concentration D of the smoke dust; testing the fluorescent intensity of SO3 in the smoke dust by using the XRF, and obtaining the ammonia content C in the smoke dust according to the SO3 fluorescent intensity in the smoke dust and the standard curve; and obtaining the escape rate M of ammonia according to the concentration D of the smoke dust and the ammonia content C in the smoke dust. The invention provides the simple and rapid method for detection of the escape rate of ammonia, wherein the method makes up the limitations of current ammonia escape on-line monitoring means in power plants, also reduces the consumption of a reducing agent and improves the operation efficiency of units.