The thermal hydraulic performances of Steam Generator (SG) under both steady and transient operationconditions are of great importance for the safety and economy in nuclear power plants. In this paper,based on our self-developed SG thermal hydraulic analysis code STAF (Steam-generator ThermalhydraulicAnalysis code based on Fluent), an improved new version STAF-CT (fully Coupling and Transient)is developed and introduced. Compared with original STAF, the new version code STAF-CT has twomain functional improvements including “Transient” and “Fully Three Dimensional Coupling” features. InSTAF-CT, a three dimensional energy transferring module is established which can achieve energy exchangecomputing function at the corresponding position between two sides of SG. The STAF-CT isvalidated against the international benchmark experiment data and the results show great agreement. Then the U-shaped SG in AP1000 nuclear power plant is modeled and simulated using STAF-CT. Theresults show that three dimensional flow fields in the primary side make significant effect on the energysource distribution between two sides. The development of code STAF-CT in this paper can provide aneffective method for further SG high fidelity research in the nuclear reactor system.