A comprehensive stochastic model for simulating microstructure formation of Ti-45 % Al(mole fraction)alloy ingot during solidification process was developed, based on a finite differential method(FDM) for macroscopic heat flow calculation and a cellular automaton(CA) technique for microscopic modeling of nucleation and growth.The formation of a shrinkage cavity at the top of ingot was taken into account. The effects of process variables such as pouring temperature and mold-preheated temperature on the microstructure formation were investigated. The calculated results indicate that the columnar zone is expanded with increasing pouring temperature in the nonlinear way and the volume fraction of equiaxed zone only slightly varies with the mold-preheated temperature.