Abstract: There generally exists a margin of strength in the joints designed by theoretical mis-match ratio.According to data of hardness test of butt joints,practical mis-match ratios and stress concentration factors are compared to determine minimum practical mis-match ratio.In terms of the principle which makes the stress concentration factor at weld root equal to the minimum practical mis-match ratio,the geometry parameters of undermatching butt joint with equal load-carrying ability under static load stress designed by yield strength mis-match ratio for high strength steel are designed by tensile strength mis-match ratio once again.This design avoids unnecessary waste and remarkably decreases the geometry parameters of joint due to taking metallurgy strengthening effect into account.