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An experimental and theoretical study of single-crystal silicon joining was undertaken by the comparison of two joining methods of eutectic joining and pre-eutectic joining. The results indicate that when holding for 30 min, the lowest joining temperatures for eutectic and pre-eutectic joining are 600 oC and 430 oC, respectively. Thermodynamic analysis shows that during the eutectic joining, the main resistance to the eutectic liquid formation is the transformation from Au and Si to Au or Si base supersaturated solid solutions (SSS). However, cladding of Au-Si eutectic layer is prefabricated in the surface to be joined by pre-eutectic joining. Since separation between Au and Si is insufficient during the cladding cooling, the supersaturate solid solution (SSS) exists inside the cladding and around the cladding/substrate interface. As a result, the re-eutectic of Au-Si during the pre-eutectic joining process is not impeded by the above-mentioned resistance, but promoted by the Gibbs free energy decrease induced by the transformation from the SSS to eutectic liquid. Therefore, the re-eutectic liquid is more prone to appear, and the joining temperature is observably reduced.