To invetigate the impurity distribution in GaAs crystal grown by horizontal Bridgman method, we constructd the mathematical model describing heat transfer, mass transfer and fluid flow n transient growth of GaAs. Galerkin finite element method and implicit time integration were used to solve the equations and simulate the transient growth. The concentration distribution is similar to the case of diffusion controlled growth when Gr - 0. With the increase of Gr the concentration profile is distroted and the minimum solute concentration appears near the interface. As solidification prosceeds, interface deflection increases steadily and transverse segregation increases until mixing by flow becomes steady. The axial segregation increases with solidification. But, with high intensity of flow axial segregation becomes steady after short transient. At small and large Gr the result showed a good agreememt with the prediction Smith and Scheil.