The invention discloses a method for preparing a ZnO/g-C3N4 composite of a core-shell structure. The method specifically includes the steps that urea is dissolved in secondary water, the materials are stirred at room temperature to obtain a clear solution, zinc oxide nano-particles are ultrasonically dispersed in the clear solution, stirring is carried out at room temperature to volatilize water, white solid powder is obtained, the obtained white solid powder is transferred into a porcelain crucible, the porcelain crucible is put in a muffle furnace in an open mode, firstly, low-temperature heat treatment is carried out for 30 min at the temperature of 125-135 DEG C, the temperature is raised to 450 DEG C, high-temperature heat treatment is carried out for 2 hours, and finally the ZnO/g-C3N4 composite of the core-shell structure is obtained. The preparation process is simple, easy to carry out, good in repeatability and capable of being used for mass production, and the prepared ZnO/g-C3N4 composite of the core-shell structure has good application prospects in the related fields such as optics, electrics and thermotics.