Abstract: The P11 heat resistant steel was welded by the narrow gap automatic TIG method, and the bead was filled using multi-layer single pass technology. Then the metallographic structures were observed and mechanical properties were tested, and the relationship between microstructure distribution characteristic and micro-hardness was particular searched. The results show that the filling bead consists of ferrite and troostite, and the coarse and fine grained regions are appeared alternately. When compared with base metal, the microstructure is finer and micro-hardness is higher. The cosmetic bead is mainly composed of coarse lath martensite and granular bainite while reticular troostite emerged, and the micro-hardness is higher than that of filling bead. In heat affected zone (HAZ) of filling bead, ferrite and pearlite with a banding distribution are observed, which is similar with the filling bead. In the thickness direction, the ultimate tensile strengths of top and bottom are almost the same, which is corresponding with microstructure distribution characteristic. And the impact tests show that absorption energy can meet all the requirements.