Abstract: Based on two fluid Euler model combining with RPI wall boiling model, the coupled heat transfer among the primary side, the tube wall and the secondary side in the subcooled boiling region of the heat transfer tube bundle of steam generator (SG) was simulated by using ANSYS CFX 12.0 code. The flow boiling phenomenon occurred in the heat transfer tube bundle of SG under trefoil orifice plate and different inlet subcooling conditions was researched. The velocity and temperature distributions of both the primary and the secondary sides, the void fraction distribution of the secondary side, the local flow characteristics around trefoil orifice plate, and the effect of different inlet subcoolings on thermal-hydraulic characteristics of SG were acquired. Numerical simulated results show that the existence of trefoil orifice plate and inlet subcooling have a significant influence on thermal-hydraulic characteristics in the subcooled boiling region of the tube bundle of SG.