The invention discloses an acid mixing system of lead-acid storage batteries. The acid mixing system comprises an acid preparing tank, a cooling device and an acid preparing pump. An acid injecting tube, an acid liquor outlet tube, an acid liquor return tube and a liquidometer are arranged on the acid preparing tank. A mixer is arranged close to the bottom in the acid preparing tank. An exhaust device is arranged on the acid injecting tube. A pressure emptying device is arranged on the liquidometer. An acid liquor concentration detecting device is arranged on the acid liquor return tube. By the arrangement of the mixer, the mixing can be uniform. By the adding of the exhaust device, air in the acid injecting tube is prevented from forming air block which blocks the smooth injection of concentrated acid. By the adding of the pressure emptying device, the accuracy of the measurement is ensured. By the arrangement of the concentration detecting device, redundant mixed cycle is avoided on the premise that the accuracy of the measurement is further ensured, and the production efficiency is ensured.