The invention discloses a bonding agent for decayed tooth filling treatment and a preparing method thereof. The bonding agent comprises, by weight, 34-52 parts of epoxy resin, 5-13 parts of sodium hexafluorisilicate, 6-19 parts of glycerophosphoric acid methacrylate, 12-17 parts of hydroxyethyl cellulose, 15-22 parts of talcum powder, 32-43 parts of glycerinum, 3-8 parts of methacrylic acid, 12-25 parts of dibutene and 6-17 parts of triethanolamine. The prepared bonding agent is high in bonding strength, good in compressive property and not prone to being dissolved in saliva.