With the development of industry, agriculture, and aquaculture, excessive ammonia nitrogen mainly involving ionic ammonia (NH₄⁺) and molecular ammonia (NH₃) has inevitable access to the aquatic environment, posing a severe threat to water safety. Photocatalytic technology shows great advantages for ammonia nitrogen removal, such as its efficiency, reusability, low cost, and environmental friendliness. In this study, CP (g-C₃N₄/CoP) composite materials, which exhibited high-efficiency ammonia nitrogen removal, were synthesized through a simple self-assembly method. For the optimal CP-10 (10% CoP) samples, the removal rate of ammonia nitrogen reached up to 94.8% within 80 min under visible light illumination. In addition, the nitrogen selectivity $S\left(N_2\right)$ is about 60% for all oxidative products. The high performance of the CP-10 photocatalysts can be ascribed to the effective separation and transmission of electron–hole pairs caused by their heterogeneous structure. This research has significance for the application of photocatalysis for the remediation of ammonia nitrogen wastewater.