Abstract: In order to study the steady thermal-hydraulic characteristics of a steam generator, a 1D simulation code based on the 4-equation drift flux model was developed. The U tube bundle space was considered to consist of the primary channel, the secondary channel and the tube wall. The flowing characteristics in sub-cooling part of the primary and secondary channels were simulated with single phase flow model, while the boiling part in the secondary channel was simulated with 4-equation drift flux model. The first-ordered upwind differencing equations were derived based on the staggered grid. An alternate iteration method of heat balance and driving force of natural circulation was implemented then. By the proposed method, the steady-thermal-hydraulic characteristics for steam generator of Qinshan 300 MW NPP, under 100%, 75%, 50%, 30%, 15% power condition, were analyzed and compared with simulated results of RELAP5. Two sets of results are in good agreement.