The invention discloses a preparation method of a composite material modified electrode for detecting lactic acid concentration, and an application. The preparation method comprises the following steps: performing loading on metal phthalocyanine and acidized carbon nanometer material by adopting ultrasonic impregnation method to synthesize poly-metal phthalocyanine-carbon nanometer composite material, and then loading the composite material suspension uniformly dispersed through ultrasonic on a glassy carbon electrode surface by adopting a dispensing method, thereby preparing a poly-metal phthalocyanine-carbon nanometer composite material modified electrode. The preparation method disclosed by the invention has excellent electrocatalytic activity for the lactic acid, and has the features of being high in sensitivity, good in stability, and strong in interference resistance, and the detection process is simple and efficient.