An efficient finite element (FE) method, multivariate quadratic regression approach, is proposed for calculating the fatigue stress spectrum of engineering structures. The calculation starts from a dynamic load history. A regression function is established as a relation from the concurrent components of structural balance force systems into the structural stress. Parameters of the relation are measured by the stress data using an elastic-plastic FE method under selected groups of balance force systems. The selected systems should cover the possible load range with a similar service condition. After passing the fit effect check, the regression equation can transfer the structural load history into the stress history at the preferred position. Application on the determination of fatigue stress spectrum for the bolster of K6 type bogie has indicated availability of the present approach. Some deficiencies have been overcome from the much time cost by transient approach and the entire elastic material relation by quasi-static approach.