Solute diffusion controlled solidification model was employed to simulate the columnar to equiaxial transition (CET) of TiAl alloy casting. The simulation results show that solute shock wave occurs when the diffusion fields of columnar and equiaxial dendrites meet. It could be one of the main factors blocking the columnar dendrite growth. Using this model, the CET in small castings was qualitatively predicted. It was found that the thermal gradient and solute undercooling ahead of the columnar tips of CET act jointly on CET, being in good agreement with experimental observations.