Abstract: Based on the tension and pulse fatigue tests,the influence of mis-match ratio on mechanical performance of undermatching flush butt joints of 10CrNi₃MoV steel was studied, and the empirical equations for the relationship between mis-match ratio and fatigue strength (or fatigue life) of joints were built. The results of experiments illustrated that the tensile strength, specific elongation and fatigue strength of flushing butt joints decreased with the decreasing mis-match ratio, and the influence on specific elongation and fatigue strength was obviously higher than that on the tensile strength. The tensile strength and fatigue strength of undermatching flush butt joints exceed their deposited metals by the metallurgic strengthening effect and constrained strength effect on weld metal. Because a low to excess of mis-match ratio welded structure lacking plasticity reserve of need, the preliminary definition of low limit of mis-match ratio was attempted under standard of the rupture models of welded joints.