The simplified calculation of the first-order longitudinal vibration period for a cable-stayed bridge is very important for the comparison of design plans and the evaluation of seismic performance. Firstly, according to the longitudinal seismic inertia force transmission of cable-stayed bridges, the double-mass model derived by flexibility method was developed to simplify the calculation of the first-order longitudinal vibration. Based on significant coupling between the longitudinal modes and vertical modes, the simplified calculation of the first-order longitudinal vibration period was then investigated by energy principle in fixed hinge cable-stayed bridges. Finally, the two formulas were evaluated by the tests on ten built-up bridges. It is concluded that these two simplified formulas were in good agreement with those predicated by finite element method. The proposed double-mass model has higher accuracy and reliability.