Abstract: Three-dimensional (3D) core nuclear design code COCO is developed in China Guangdong Nuclear Power Holding Corporation (CGNPC), and the basic functions of COCO include core power distribution calculation, fine core power reconstruction, critical boron density search, critical control rod position search, and nuclear density calculation, etc. The COCO employed Green’s function nodal method as the key solver to calculate the core power distribution, and the single-channel model and the rod heat transfer model were used to calculate the moderator density and fuel temperature. The calculation capacity from the beginning of cycle to the end of cycle was achieved in the COCO code. Comparing with the reference code’s values, the theoretical models and coupling processes used in the COCO is proved to be correct with sufficient calculating accuracy which can meet requirements for engineering design.