Abstract: A coupled three dimensional neutronics/thermal-hydraulics code STTA（SCWR Three dimensional Transient Analysis code） is developed for SCWR core transient analysis. Nodal Green’s Function Method based on the second boundary condition（NGFMN_K） is used for solving the transient neutron diffusion equation. The SCWR sub-channel code ATHAS is integrated into NGFMN_K by the serial integration coupling approach. The NEACRP-L-335 PWR benchmark problem and SCWR rod ejection problems are studied to verify STTA. Numerical results show that the PWR solution of STTA agrees well with the reference solutions, and the SCWR solution is reasonable. The coupled code can be well applied to the core transients and accidents analysis with 3-D core model during both subcritical pressure and supercritical pressure operation.