With the aim of improving the fatigue properties of Mg alloy welded joints under cyclic loading, the effects of laser bionic treatment and ultrasonic impact bionic treatment on the fatigue crack growth (FCG) behavior of AZ31B Mg alloy TIG-welded joints were studied and compared. The results show that bionic treatment refines the grains on the joint surface and improves the microhardness. In the crack stable growth stage, both bionic samples exhibit a lower FCG rate and a higher FCG resistance. The two bionic treatment methods reduce the probability of crack initiation and partially promote crack deflection, providing a new approach for improving the FCG behavior of welded joints.