Stress intensity factors of thin AZ31B magnesium alloy sheet under biaxial tension loading were analyzed by modified Dugdale model.K-values with crack angle of 90°obviously show that there is no influence of the loading condition in Mode-I.In the 45°case,K Ⅰ values are obtained within 10%errors when they are calculated by modified Dugdale model under biaxial loading.It is concluded that the modified Dugdale model is one of effective ways to evaluate stress intensity factor of AZ31 magnesium alloy sheet appropriately. Stress intensity factors of thin AZ31B magnesium alloy sheet under biaxial tension loading were analyzed by modified Dugdale model. K-values ​​with crack angle of 90 ° obviously show that there is no influence of the loading condition in Mode-I.In the 45 ° case , K I values ​​are obtained within 10% errors when they are calculated by modified Dugdale model under biaxial loading. It is said that the modified Dugdale model is one of the effective ways to evaluate the stress intensity factor of AZ31 magnesium alloy sheet appropriately.