The invention relates to an in-situ preparation method of a zinc phthalocyanine/carbon nanotube composite catalyst based on a solvothermal method. The in-situ preparation method comprises the steps of pretreating a carbon nanotube: adding concentrated nitric acid to the carbon nanotube, flowing back, heating, mixing, cooling, washing with water to be neutral, performing suction filtration, and drying; preparing the zinc phthalocyanine/carbon nanotube composite catalyst: adding the carbon nanotube, phthalonitrile, octan zinecnaty, 1,8-diazabicyclo [5,4,0] hendecane-7-alkene and the carbon nanotube to a beaker, adding a solvent, mixing for 30 minutes, and then pouring into a reaction still for solvothermal reaction. The in-situ preparation method has the beneficial effects that the used solvent is low in price and is easy to obtain, and no environmental pollution is generated; the preparation method of the zinc phthalocyanine/carbon nanotube is simple, the reaction time is short, the aftertreatment is easy, and industrial production is facilitated; effective degradation of pollutants malachite green is realized, and the photocatalytic activity is obviously better than that of pure zinc phthalocyanine particles.