The effect of ultrasonic impact treatment (UIT) on very high cycle fatigue properties of MB8 magnesium alloy welded joint was investigated by ultrasonic fatigue test. At the same time, the mechanism of UIT to improve the very high cycle fatigue properties of MB8 magnesium alloy welded joints was investigated from three factors, such as stress concentration, residual stress and grain refinement. The results show that the S-N curve of the fatigue life, in 1× 108 cycles, the fatigue strength of as-welded specimen is 31.62MPa , the fatigue strength of treated specimen is 39.81MPa, and the fatigue strength of treated specimen is 26% higher than that of the as-welded. This shows that UIT can obviously improve the very high cycle fatigue properties of MB8 magnesium alloy welded joint. The weld toe stress concentration factor of as-welded specimen Kt1 is equal to 1.95, the weld toe stress concentration factor of treated specimen Kt2 is equal to 1.67, and stress concentration factor is reduced by 14.4%. The UIT can reduce the stress concentration of the weld toe. After UIT, the stress at the toe of the specimen changes from residual tensile stress to residual compressive stress. UIT refine surface grain of welded toe to improve very high cycle fatigue properties of MB8 magnesium alloy welded joint.