The invention discloses a method for detecting the deposition blocking degree of a ferromagnetic matter in an austenite stainless steel bent tube for a boiler. The method comprises the following steps: selecting a bent tube identical to a to-be-detected austenite stainless steel bent tube for the boiler as a sample tube, applying a stable constant magnetic field on the right upper part, the right side part and the right lower part of the outer wall of the sample tube added with the ferromagnetic matter, measuring the corresponding magnetic flux value, respectively recording the average value of the magnetic flux corresponding to the accumulation blocking sectional area of the ferromagnetic matter in the sample tube, creating a graph of relation, using the same method to apply a stable constant magnetic field on the to-be-detected tube, measuring the average value Theta of the magnetic flux of the ferromagnetic matter in the to-be-detected sample tube, contrasting the average value Theta of the magnetic flux of the ferromagnetic matter in the to-be-detected sample tube with the graph of relation, finding out the accumulation blocking sectional area of the ferromagnetic matter corresponding to the magnetic flux identical to the average value Theta of the above magnetic flux, and taking the accumulation blocking sectional area of the ferromagnetic matter as the accumulation blocking sectional area S of the ferromagnetic matter in the to-be-detected tube on the site. According to the method, the outer wall of a pipeline does not need to be specially treated during the detection, and the method has the characteristics of simplicity, safety, reliability and high sensibility.