The invention relates to a soldering flux and in particular relates to a soldering flux of a busbar of a lead-acid storage battery. The soldering flux comprises the following components in percentage by weight: 8-15% of metaphosphoric acid, 3-8% of phosphate, 10-15% of ethyl alcohol, 1-5% of carmine and the rest of water, wherein the total weight percentage is 100%. The invention aims at providing a soldering flux of a busbar of a lead-acid storage battery and solving the problems that a plate lug is undesirably melted together with a lead solution, a plurality bubbles are generated after the lead solution is cooled and the lead solution is not adhered to the plate lug.