Abstract: The accuracy of fatigue life predict of welded structures based on traditional fatigue evaluation methods and some standards is insufficient which caused by the singularity of stress at the crack tip, and it affects the reliability of engineering structures. Because crack extension is caused by external load driving on the section where the weld toe or root is located, the structural stress method is proposed based on the force balance relationship at the weld seam. This method converts the unit node forces into line forces, and it comprehensively considers different welded joint types, plate thickness and loading patterns.The fracture mechanics theory is applied to unify the whole process from small complex cracks extension to fatigue failure, and the main S-N_d curve fatigue assessment expression is derived in terms of equivalent structural stress amplitude.The structural stress method with grid-insensitive mechanical characteristics is proved by using different cell simulation specimens, and the weak position of the weld and the actual fatigue crack are consistent with the test data, which verifies that the structural stress method has high fatigue prediction accuracy.The welded steel-tube concrete structure verifies the validity and reliability of the structural stress method for fatigue life assessment.