A method is described of preparing a tin-silver (Sn—Ag) alloy plating solution containing methanesulfonic acid tin, methanesulfonic acid silver, methanesulfonic acid, and an additive, wherein the method includes: (a) eliminating impurities such as released chloride compounds and released sulfur compounds, which are present in the methanesulfonic acid, (b) preparing the methanesulfonic acid tin and the methanesulfonic acid silver by dissolving, through an electrolytic process, tin and silver in the methanesulfonic acid from which the impurities are eliminated; (c) producing a mixture solution by adding the methanesulfonic acid, the methanesulfonic acid tin, the methanesulfonic acid silver, and the additive; and (d) filtering the mixture solution. And by the method thereof, current efficiency may be increased and a desirable plating film may be maintained by eliminating the impurities from the methanesulfonic acid used as a base material and preparing the Sn—Ag alloy plating solution.