A coupled neutronics/thermal-hydraulics three dimensional code system SNTA is developed for SCWR core steady state analysis by modular coupling the improved neutronics nodal methodological code and SCWR thermal-hydraulic sub-channel code. The appropriate outer iteration coupling method and self-adaptive relaxation factor are proposed for enhancing convergence, stability and efficiency of coupled N/T calculation. The problem of CSR1000 core is studied to verify SNTA. The results calculated by SNTA are agreed well with those by CASIR and SRAC. SNTA is more efficient than CASIR and SRAC whose neutronics modules are based on the Finite Difference Method. The numeric results show that SNTA can be well applied to SCWR core steady state analysis and core concept design.