Abstract: The torsional inertia is an important parameter to measure the mechanical performance of long-span plate truss girders. As a detailed finite element model of plate truss girders is complex and the model test is costly, a succinct and reliable computational method is a better choice. First, the analytic formula of equivalent plate thickness of the main truss, taking into account the chord framing effects of the truss structure, was derived based on the shear stiffness equality principle. Secondly, the analytic formula of equivalent plate thickness of the orthogonal plate was improved based on the consistency of horizontal shear deformation between the bridge panel and the U-rib, and the calculation formula of torsional inertia of the plate truss girder was presented. Finally, the detailed finite element model of a practical plate truss girder was taken as an example to prove the validity of the computational method. The results show that the proposed method for calculating the torsional inertia of the plate truss girder has a high degree of accuracy.