Abstract: Various suspensions were made by four kinds of solutions mixing with different Si3N4 powders contents; the Si3N4 coatings, which used for polycrystalline silicon purification, were prepared in quartz crucibles inwall; the adhesive area between polycrystalline silicon ingots and crucible, microcrack morphology on the ingots surface and thickness of reaction layers were analyzed by SEM, EPMA, and so on. The process parameters prepared coating matching with best polycrystalline silicon knockout were got and the reaction mechanism between Si3N4 coating and melt silicon in melting process was analyzed. The results show that the coating was prepared by spraying the suspensions including the ethyalcohd solution containing 8wt% PVP, 60wt% SiC on crucibles inwall, and the coating sintered at 210 ℃ for 15 min was not decompounded, the crucible inwall was integrated, and the knockout effect of the ingot was best. The dissolution of Si3N4 coating aggravated with the increase of smelting temperature. The continuous layer, which reduced the possibility of impurity diffusing from crucible and coating to silicon ingot interior, was consisted of large Si3N4 particles on the interface of coating and silicon ingot.