Here, we report a simple hydrothermal method and in situ calcination to synthesize ZnO/K-CNx heterostructure nanocomposites. The optimized ZnO/K-CN0.5 exhibits excellent photocatalytic degradation and adsorption efficiency of tetracycline under simulated sunlight, and the degradation rate reaches 90% (the adsorption removal rate is 50%). Moreover, ZnO/K-CN0.5 also has high stability and recyclability. The excellent photocatalytic activity can be attributed to the following factors. (1) The K-doped exfoliated g-C3N4 nanosheet (K-CN) and the ZnO nanorod form a unique type-II heterojunction that promotes the separation of photogenerated carriers and provides very short photogenerated carrier diffusion distances. (2) The surface zeta-potential can enhance the adsorption capacity of tetracycline through electrostatic attraction and repulsion, resulting in synergistic effects of photocatalysis and adsorption. This work highlights the important role of integrating doping and exfoliation techniques into type-II heterojunctions to form nanocomposite photocatalyst materials, which have great potential in environmental water pollution treatment and the maintenance of human health.