The vertical bending stiffness assessment of the cable-stayed bridge has been a hot research issue in the dynamics of bridge. Based on the mechanical characteristics of stay cable supporting deck，a new evaluation method was proposed for the vertical bending stiffness of a floating single-tower cable-stayed bridge. Firstly，a new dynamic model (i.e.，triple-beam with discrete springs) of the floating single-tower cable-stayed bridge was proposed and its corresponding dynamics theory was derived. In the proposed model，the stay cables were simplified as springs without mass and the single-tower was regarded as an Euler-Bernoulli beam with consideration of axial force. At the same time，the deck was divided into two segments at its intersection with tower，hence the deck was regarded as two Euler-Bernoulli beams. The eigenvalue and eigenvector of the dynamic system were then solved by the transfer matrix method，which was used for the evaluation of vertical stiffness of the floating single-tower cable-stayed bridge. Finally，the case study and its comparison with results obtained by the finite element method show that the evaluation method proposed is of high precision and efficiency，and can be used for the calculation and stiffness evaluation in engineering design.