The invention relates to a gold-tin eutectic solder (AuSn20) electroplating solution and a preparation method thereof. The solution is characterized in that: novel gold potassium citrate is used for replacing traditional gold potassium cyanide, such that the pollution to the environment is reduced; bivalent tin is complexed by using another complexing agent, such that the stability of tin salt is improved. Every liter of the electroplating solution comprises 1-20g of gold salt, 1-20g of tin salt, 10-50g of a buffering agent, 10-50g of a complexing agent, 0.1-10g of a brightening agent, and 0.1-5g of an anti-oxidant. The electroplating solution provided by the invention is environment-friendly, and has relatively wide range of available current density. The obtained plating coat has good brightness, the gold and tin components are precisely controlled, and the melting point is 278-282 DEG C. The electroplating solution is suitable for welding and packaging of photoelectric devices in fields with relatively high requirements on reliability, such as communication, satellite, remote sensing, radar, automobile, aviation, and the like.