The invention discloses a method for accelerating phase changing of bainite in steel through in-situ nanometer AlN heterogeneous nucleation. The method is mainly characterized in that Al with the weight percent ranging from 0.1 to 0.2 and 100 ppm to 200 ppm of N are added into alloy steel, wherein the carbon content of the alloy steel ranges from 0.4 weight percent to 1.2 weight percent, the silicon content of the alloy steel ranges from 1.5 weight percent to 2.8 weight percent, and MnCrNiMo is contained in the alloy steel; and in the heat deformation and heat treatment process of the steel, AlN particles which are evenly distributed with the size ranging from 20 nm to 100 nm and the volume fraction ranging from 0.1 vol.% to 0.2 vol.% are separated out from the steel in an in-situ manner, the bainite phase changing time ranges from 0.5 h to 5 h, and a bainite batten with the thickness ranging from 30 nm to 100 nm is obtained. By means of the method, the bainite phase changing time is shortened by 1 time to 10 times; the bainite ferrite batten and a residual austenite film are smaller; and the bainite steel has more excellent welding performance.