In this paper,introduced the equipment and the principle parameters in Ultra-Narrow Gap MAG Welding process applied for 34Cr2Ni2Mo and 35 CrMoV dissimilar steel welding,and then analyzed the microstructures and the mechanical performance via metallographic observation and a series of mechanical measurements of the welded joint. Observed that fine grain microstructures existed in weld bead and HAZ,and a little bigger grain microstructures appeared in a very narrow over-heated zone,the maximum hardness was within the allowable range. In 34Cr2Ni2 Mo side HAZ,a few slag inclusion defects were found. The results of mechanical measurements show the mechanical performance of the welded non-defect founded joint which is much better than base metals. The use of ultra narrow gap MAG welding with low heat input welding technology is conducive to the formation of fine grains in the weld and heat affected zone,through the fine grain strengthening improve the tensile strength of the weld metal greatly,easy to obtain excellent welding joint toughness.