The invention relates to a co-deposition electroplating method for preparing Au-Sn alloy on an electroplating substrate and a cyanogen-free Au-Sn alloy electrolyte. The wave forms of electroplating current pulses are forward variable-amplitude pulses, namely that two different forward square wave pulses exist in one period, and the peak values and the conducting time of the electroplating current pulses respectively correspond to peak value current density and time which are required for generating Au5Sn and an Au-Sn alloy phase in a plating layer. In the electrolyte, sodium sulfite is adopted as a main complexing agent and ethylene diamine tetraacetic acid is adopted as an auxiliary complexing agent for the gold ion complexing agent, potassium pyrophosphate is used as a tin ion complexing agent, hydroquinone is used as an antioxidant, and the pH value of the electrolyte is from 8 to 9. When the method is used for preparing Au-30at.%Sn eutectic plated layer through electroplating, the plating speed reaches 13 mum/hour, the electrolyte is stable, the operation is simple, and the gold and tin content in the plated layer is easy to control. The method can be applied in microelectronics and photoelectron industry for connecting and encapsulating LED chips, connecting flip chips, forming welding pads or patterns on the surface of semiconductor devices or similar devices, etc.