Abstract: In an effort to provide technical references for thermo-hydraulic design of the proposed supercritical water-cooled solid breeder blanket of fusion reactor, the unidirectional fluid-solid coupling numerical analysis for the first wall with China low activation Martensitic steel (CLAM) as structural material was carried out under its operating conditions, using numerical computation software ANSYS. The temperature and stress distribution in the first wall of the two structural materials, namely, CLAM and F82H, was comparatively discussed. The effects of different cooling duct geometrical shapes (rectangular/round flow area) and of different physical dimensions (tube diameter and minimum wall thickness) on the temperature and stress field were considered. The results show that both the maximum temperature and the maximum stress in the first wall with CLAM as structural material are of the same order but a bit higher than those in the same heat load situation with F82H. For the case of CLAM first wall with rectangular coolant duct, heat transfer enhancement is observed in the corner region of the duct as well as, however, stress concentration. It is therefore recommended that a comprehensive balanced consideration should be taken on both the heat transfer and stress performance in optimum design. Furthermore, it is concluded that such strategies as increasing duct diameter and decreasing minimum wall thickness are both in favor of a good heat transfer performance.