Influence of pressure on solidification microstructure of FeSi2 alloy is investigated. Different from the eutectic structure of the sample solidified at normal pressure, the microstructure of the sample solidified at high pressure is composed of dendritic primary phase and anomalous eutectic. The significant change in microstructure results from the change of phase diagram and solute atom diffusion induced by high pressure. By introducing the pressure parameter, the constitution undercooling criterion is attained. Formation mechanism of dendritic primary phase under high pressure is analysed according to the constitution criterion.