A method for cultivating algae on the surface of a composite insulating material comprises a step S1 of collecting mixed dirt containing algae on the surface of a composite insulating material where wild algae grow, and removing ash contents and then using culture solution to store the mixed dirt; a step S2 of removing soil in the mixed dirt, fungi and salt in the culture solution, purifying the algae, obtaining purified alga solution, and adding culture solution to perform culture; a step S3 of extracting extracellular secretion (EOM) from the algae having undergone inactivation treatment, and laying the extracellular secretion on the surface of the composite insulating material prepared well; a step S4 of measuring the density of alga cells of the alga solution obtained through the step S2, taking a fixed amount of alga solution according to the number of required cells, and inoculating the algae to the surface of the composite insulating material treated through the step S3, so that the algae grow on the surface of the composite insulating material. According to the method for cultivating algae on the surface of the composite insulating material, the growth conditions of the algae on the site can reappear precisely, samples are prepared for external insulation tests, and the method is favorable for implementing related tests.