The undercooled solidification experiment of Ag-Cu/Co-Sn eutectic alloy was performed by means of molten glass purification and cyclic overheating, and the effects of undercooling and minor Nb addition on the formation of anomalous eutectic were systematically studied. The results show that anomalous eutectic is formed by remelting in the eutectic dendrite during rapid solidification. With the increase of undercooling, the morphology of eutectic phase changes from vermicular to spherical particles. The solid particles formed by remelting will be used as the substrate for the nucleation of residual liquid phase. The nucleation of two-phase in Ag-Cu eutectic is non reciprocal. With the addition of Nb to Co-Sn eutectic alloy, the critical undercooling of anomalous eutectic formation inside the sample decreases from 23 K to 15 K, and from 45 K to 30 K in the surface microstructure. Because the sample surface contact with the crucible wall which is conducive to the dissipation of latent heat of solidification, the critical undercooling of anomalous eutectic formation is higher. The duration of slow solidification stage on the temperature recalescence curve of Ag-Cu eutectic alloy is longer than that of Co-Sn eutectic alloy.