The invention belongs to the technical field of biosensors, and particularly relates to a lactic acid biosensor which comprises a substrate, a carbon working electrode, a carbon counter electrode, a reference electrode and a lactic acid sensing film, wherein the carbon working electrode is an aminated graphite printing electrode, the lactic acid sensing film is formed on the surface of the carbon working electrode after lactic acid oxidase for establishing an electron transfer network is chemically crosslinked by a difunctional chemical crosslinking agent, and the lactic acid oxidase for establishing the electron transfer network is prepared by modifying electrochemically activated lactic acid oxidase by a carboxylated nano material. The electrochemical activation technology of the lactate oxidase is adopted, so that the lactate oxidase can perform direct electron exchange with the electrode, the sensitivity, accuracy, reproducibility, stability, specificity and anti-interference capability of dynamic detection of lactic acid can be improved, the service life of the implantable continuous lactic acid monitoring system is prolonged, and meanwhile, the cost of the lactic acid biosensor is greatly reduced.