A hydrothermal preparation method of a carbon quantum dot used as a visible light catalytic photosensitizer is characterized by comprising the following steps: taking larches as raw materials; carrying out high-temperature high-pressure hydrothermal process treatment on the raw materials; centrifugally filtering and removing a dark brown solid product to obtain a light yellow liquid product; centrifuging the liquid product at high speed; controlling centrifugal rotation speed and centrifugal time; removing insoluble small particle impurities; carrying out dialysis by using a certain molecular weight of dialysis bag to remove unreacted sugars, salt contents and the like; then carrying out concentration to obtain a target product, namely a water-soluble fluorescent carbon quantum dot. The water-soluble fluorescent carbon quantum dot is compounded with TiO2 to prepare a photocatalytic composite system; by taking antibiotics as models, photocatalytic degradation performance of the composite system on the model and interaction efficiency of the carbon dot used as the photosensitizer are inspected. The operation process has the main characteristics that larches with rich resources in Heilongjiang province are used as raw materials; the raw materials are environment-friendly, are low in cost and are easy to get; hydrothermal reaction is low in temperature and low in pressure, addition of passivating agents such as acid, alkali and salt is not needed, the product is water-soluble and has a certain fluorescent effect, the reaction process is simple to operate, and the cost is low.