The invention discloses a method for controlling non-metallic inclusions in steel, and belongs to the field of steelmaking and refining control. Two-section calcium treatment and soft blow are adopted during the refining, and the calcium treatment and the soft blow are performed at the end of LF refining and RH vacuum treatment respectively, so that the non-metallic inclusions in a casting blank are classified into two types: 70 to 90 percent of non-metallic inclusions with CaS as main components, and 10 to 30 percent of oxide type non-metallic inclusions, wherein all the non-metallic inclusions in the casting blank are spherical; the particle diameters of all the non-metallic inclusions in the casting blank are controlled within a range of 0 to 5 microns; and the number of the non-metallic inclusions with the diameter of greater than 3 microns in the casting blank is controlled within the range of 0 to 5/cm<2>. Thus, the problem that the calcium aluminate type non-metallic inclusion produced after the calcium treatment cannot be discharged out of molten steel due to insufficient time, so the content of calcium aluminate type non-metallic inclusions in the casting blank is high is solved. The rejection rate caused by super-standard non-metallic inclusion of the low alloy structural steel is reduced.