The invention relates to a tin plating method for a copper bar. The tin plating method comprises the following steps: 1) carrying out polishing treatment on the surface of the copper bar by virtue ofabrasive paper, and treating the copper bar for 10-20min by virtue of steam at 100-110 DEG C; 2) placing the copper bar subjected to the steam treatment in an alkali liquid at 70-75 DEG C and carryingout alkali washing for 30-40min, and then washing away the residual alkali liquid on the surface of the copper bar by virtue of clear water; 3) placing the copper bar subjected to the alkali washingin an acid liquid at 30-35 DEG C and carrying out acid washing for 10-15min, and then washing away the residual acid liquid on the surface of the copper bar by virtue of clear water; 4) carrying out drying treatment on the copper bar subjected to the acid washing; 5) placing the copper bar treated in the step 4) in an electroplating bath and carrying out electroplating, wherein the formula of theelectroplating is as follows: 70-80g/L of stannous sulfate, 25-30ml/L of sulfuric acid and 3.5-5ml/L of S-150B gloss agent; and 6) carrying out heat treatment on the electroplated copper bar at 800-900 DEG C to obtain a finished product. The tin plating method disclosed by the invention is high in tin plating efficiency and quality; the surface of a tin plating layer of the copper bar is high in brightness and smooth; and the plating layer is uniform and fine in crystallization, and high in corrosion resistance.