Abstract: First, the hot spot stress concentration factor (Khs ) of butt joints, transverse non-load-carrying cruciform joints and non-load-carrying longitudinal fillet plate joints are calculated using ANSYS. Then, according to the relevant fatigue tests results of ultrasonic impact treated welded joints of Q235B and Q345B, the corrected hot spot stress design S-N curve by stress ratio, thickness and geometric irregularities of ultrasonic impact treated welded joints is obtained. Analysis shows that the dispersion of data points of the hot spot stress design S-N curve of ultrasonic impact treated butt welds, transverse non-load-carrying cruciform joints and non-load-carrying longitudinal fillet welds of medium and low strength steel reduced and can be expressed with one design S-N curve independent of the form of joint. The recommended m=10.0 is also applied for hot spot stress fatigue design and the fatigue strength level of the three types of welded joint above-mentioned can uniformly applied to FAT135 at 2×106 times.