The purpose of this paper is to present a simplified formula of natural frequencies for the vertical flexural vibration of 3 span continuous cable-stayed bridge. The proposed formula is derived from the theory of beam on elastic support in dynamic behaviour. A series of numerical examples are presented to study the validity and applicability of this formula. The computed frequencies for the 1st symmetric mode and the 1st anti symmetric mode are closed to the results from the finite element analysis. The results presented may be useful to examine the dynamic stability of cable-stayed bridge in preliminary design.