In order to deeply understand the mechanisms responsible for the grain structure evolution duringdirectional solidification of Ti-46at. % alloy，characteristics of grain structure were simulated using CAFE model( Cellular Automaton-Finite Element) . CAFE model was validated by comparisons with microgravity experimentsfrom a reference. Influences of nucleation undercooling and natural convection intensity on columnar-to-equiaxedtransition ( CET) ，grain size，grain elongation and segregation were numerically investigated. Simulated resultsshow that a fully equiaxed grain structure is obtained by lowering the nucleation undercooling. Increasing flowintensity aggravates intergranular segregation，but barely has effects on variations of grain structure morphology anddistributions of grain size and elongation. With a high nucleation undercooling，CET occurs. By strengthening fluidflow，the occurrence of CET is retarded，a mixed structure of elongated and equiaxed grains is formed，both grainsizes and elongations are increased and segregation is enhanced. CAFE model can be used to predict grain structureevolution during solidification of Ti-46at. %Al and to analyze the controlling mechanisms.