Abstract: Directional solidification structure was studied under Alternating Current (AC) process. The result indicated that AC field had a refining effect on solidification structure, the effect became more obvious at a higher current, and AC field reduced the solute concentration at solidification interface. Analysis of all field effects showed that interface stability increased with the reduction of interface chemical potential, but interface layer attenuated and interface stability reduced in the situation of increased interface energy, small activity coefficient and oscillation at interface solute, accordingly the stability of cystiform crystals was restrained and columnar cystiform crystals turned refined ones.