The welding process of transverse cross joints was simulated by temperature field and welding residual stress fields in AZ31B magnesium alloy. On the basis of Dang Van criterion, considering the effect of welding residual stress, the critical plane method was used to assess the fatigue properties of the transverse cross joints in AZ31B magnesium alloy, and then the results were compared with the conventional fatigue test. The results show that the parameters for steel recommended by the French Welding Institute and for aluminum alloy recommended by investigators in china are all not suitable for AZ31B magnesium alloy which need to be amended as α*=0.5, and β*=55 MPa. Furthermore, the Dang Van curve is fitted and the result is consistent with the fatigue test, and the modified Dang Van criterion could assess the transverse cross joints in AZ31B magnesium alloy reasonably. According to the stress concentration, the possible failure position of specimens could be prognosticated