The effects of titanium modification, which was added along with the pouring liquid in ladles, on the shrinkage porosity and cavity of cast steel ZG45 smelted in intermediate frequency induction furnace were studied. The metal liquid fluidity was measured by casting spiral samples. 0.2 wt.% titanium modification achieved the highest fluidity for steel liquid. Proper content of titanium could effectively increase the volume of concentrated shrinkage cavity, and restrain the forming of shrinkage porosity. By scanning electron microscope (SEM) and energy disperse spectroscope (EDS) analysis, it was found that TiC, acting as heterogeneous nuclei, could refine the structure grains, diminish the strong dendrite, and increase the fluidity and feeding capacity of steel liquid. However, overdoes titanium modification easily caused mass oxide TiO2, which kept solid phases with high meting point in metal liquid to increase its viscosity and decrease the fluidity. Meanwhile, these solid state titanium compounds enlarged the composition supercooling, so the metal liquid changed from sequential solidification to simultaneous solidification, resulting in inadequate metal liquid feeding, less oncentrated shrinkage cavity and serious shrinkage porosity.