Five lightweight castables containing different TiO₂ additions were prepared by using porous corundum-spinel (MgAl₂O₄) ceramics as aggregate. Corrosion of the lightweight castables by ladle slag was conducted using a static crucible test. The effect of the TiO₂ addition on the slag resistances of the lightweight corundum-spinel castables was investigated through SEM, EDS and FactSage software, etc. It was found that TiO₂ additive promoted the sintering degree of the castables, decreased the average pore size, the apparent porosity, and the contact area between the castables and slag, and then enhanced the corrosion resistance. However, TiO₂ additive decreased the viscosity of the penetrated slag simultaneously, and then deteriorated the penetration resistance.