To solve the problem of separation of Si crystal with melting agent, the stability of solid-liquid interface was controlled during Si crystal growth in Al-Si melt by combining electromagnetic and thermal fields, and the functional mechanism of physic fields during the crystallization analyzed. The result showed that with increasing of Si content in Al-Si melt, the occurrence of compositional supercooling phenomenon was more difficult, and the solid-liquid interface was more stable, which was beneficial to the densification of Si crystals. When the inner diameter of crucible was reduced from 3 to 1 cm, the temperature distribution in the alloy melt changed, and the curvature of solid-liquid interface varied from 16.7 to 125, which resulted in denser Si crystals, enhanced separation efficiency of Si crystals with melting agent, and reduced existing portion of Si crystals from 0.57 to 0.42. The slower the lowering rate, the stabler the solid-liquid interface, and the better the in situ separation effect of Si crystals with melting agent. When the lowering rate was 0.05 mm/min, the content of Si crystals reached 99.9% at the bottom of sample, while the Si crystals could hardly be found on the top. Electromagnetic stirring could strengthen the flow and mass transfer in the melt, so the Si content in the frontier of solid-liquid interface was increased and the interfacial stability enhanced.