Abstract: Based on a systematic investigation of typical beam bridge overturning accidents, this paper redefined the concept of overturning considering the gradual failure process of torsion-resistant system of beam bridges, and established a primary structure involving geometric nonlinearity. Correspondingly, a new calculation method considering the most unfavorable load distributions was derived, which was validated by experimental tests and numerical analyses. The results indicate that: The distortion of the cross-section is less than 1% of the total girder rotation during the overturning process of the straight girder. And the lateral rotating angle depends on the difference between vertical displacements of the middle and the end supports caused by the flexural deformation. The anti-overturning capacity is enhanced by approximately 20% due to the favorable overturning resistance provided by the effects of support size and geometric nonlinearity in lateral rotation. And these results agree well with the experimental results, and are more consistent with the actual anti-overturning situation of the structure. According to the results of typical cases, although the overturning is influenced by several factors (such as settlement, prestressing, temperature gradient, support size), it remains conservative to calculate the anti-overturning capacity of straight girder solely based on vehicle loads.