Nucleating mechanism and controlling methods of Al2O3-TiO2 inclusion formed during steel cooling process is analyzed in this paper. According to the theory of nucleation, only TiO2 could satisfy the critical requirement of homogeneous nucleation to solidify stably when the temperature declines to less than 1863K. Furthermore, the generation of Al2O3 adhering on the surface of solid TiO2 is going to take place through heterogeneous nucleation, and then Al2O3 interacts with TiO2 to form Al2O3-TiO2 complex oxide. There would be no Al2O3-TiO2 complex oxide or much less Al2O3 on condition that the wTi·wO2 is less than 3.96×10-8. Both deeply deoxygenating treatments and controlling the mass of Ti-Fe alloy added into steel are effective means for improving steel quality.