NOVELTY - Preparing recyclable photocatalyst, comprises e.g. putting x g of carbon grains with a size of less than 10-25 nm or a thickness of less than 10C of graphene in 0.5-5 mole/l of nitric acid and backflow for 1-10 hours to obtain v l of carbon nanoparticles suspension, adding y g of sodium alga acid in carbon nanoparticles suspension and then strongly stirring to obtain the mixed liquor, where the ratio of x and v is 1:0.1-5 and the ratio of v and y is 1:4-50.    USE - The recyclable photocatalyst is useful for converting solar energy into chemical energy and for degradation or transforming harmful substance of environment and producing hydrogen energy.    ADVANTAGE - The recyclable photocatalyst can be prepared in a convenient manner.    DETAILED DESCRIPTION - Preparing recyclable photocatalyst, comprises (a) putting x g of carbon grains with a size of less than 10-25 nm or a thickness of less than 10C of graphene in 0.5-5 mole/l of nitric acid and backflow for 1-10 hours to obtain v l of carbon nanoparticles suspension, adding y g of sodium alga acid in carbon nanoparticles suspension and then strongly stirring to obtain the mixed liquor, where the ratio of x and v is 1:0.1-5 and the ratio of v and y is 1:4-50, (b) pouring a l of the mixed liquor into container until the liquid level is 4-20 mm, spraying b l of 0.2-5 mole/l of copper chloride solution on the surface of mixed liquor, adding alginic acid radical ions and cupric ions in the mixed liquor to produce gel, contracting along the permeating way of cupric ions to form through-holes and then reacting the copper gel with through-hole structure of alginic acid for 1-5 hours, where the ratio of a and b is 1:1-12, and (c) immersing the through-hole structure of copper alginate gel in 0.2-5 mole/l of sodium hydroxide solution or potassium hydroxide solution for 1-5 hours, separating the cupric ions from the copper alginate gel and then combining with hydroxyl ions to form copper hydroxide sediment depending on the through-hole structure to restore, spraying 0.05-2 mole/l of hydroxylamine hydrochloride solution or sodium borohydride solution at constant temperature on the surface of the copper alginate gel with through hole structure, reverting the cupric ions in the copper hydroxide sediment to cuprous ions to obtain cuprous oxide and then nucleating on the surface of surrounding carbon nanoparticles to form a composite structure of nanoparticle surface modifying with cuprous oxide nanocrystalline and grafting the inner wall of through-hole structure after reacting for 10-60 minutes to obtain photocatalyst.