The invention provides an active agent for repairing a lead-acid battery of an electric vehicle and a modulation method of the active agent, and relates to the technical field of battery repairing. The active agent comprises the following components in parts by weight: 10-12 parts of diammonium hydrogen phosphate, 6-8 parts of potassium persulfate, 2-4 parts of sodium pyrophosphate, 4-6 parts of sodium thiosulfate, 0.2-1 part of zinc sulfate, 0.2-0.4 part of manganese dioxide, 0.02-0.08 part of bismuth trioxide, 0.001-0.01 part of thulium oxide, 1-2 parts of activated carbon and 100-200 parts of distilled water in sequence. The method comprises the steps of firstly pouring the distilled water into a plastic container, fully mixing the diammonium hydrogen phosphate, the potassium persulfate, the sodium pyrophosphate, the sodium thiosulfate and the zinc sulfate and then pouring a mixture into the distilled water in the container; and carrying out cooling until the temperature of the solution returns to 25 DEG C and adding the weighed manganese dioxide, bismuth trioxide, thulium oxide and activated carbon according to the parts by weight. The battery is good in performance after being repaired, long in service life, good in low temperature resistance and high in efficiency.