In practical application, when the test weight is insufficient, it is urgent to solve the safety and reliability problems of crane structure. The deflection value is one of the key inspection indices of crane girder under rated loading in overall test. Therefore, it is very important that the influence of structure parameters on the mid-span deflection is quantified. For large-span and heavy-duty gantry cranes, the effect of rigid-soft legs on mid-span deflection was not considered in the past. So a mathematical model was established for universal large-span and heavy-duty gantry cranes. Considering the coupling effect of the bending moment of girder and legs, the analytical solution was derived and obtained on the basis of the variational principle. Then, the experimental model was designed and experiments were performed on the WEW-600B type testing machine. The experiment data show that the analytical solution of the gantry-frame structure deflection is more accurate than the pre-existing calculation method, which provides an analytical tool for the design and inspection of gantry structures.