In order to facilitate the calculation of the vertical natural frequency of the asymmetric suspension bridge of the two towers, the Rayleigh method is used to derive the approximate expression of the first order bending vibration frequency under the influence of the stiffness of the main tower. The influence of the stiffness of the expression, and finally the feasibility of this formula is verified. The results showed that the stiffness of the main tower has an effect on the first-order symmetrical vertical bending frequency of the structural system and has no effect on the first-order anti-symmetrical vertical bending frequency. The main tower stiffness can be calculated by the influence coefficient of the main tower. The degree of error between the solution and the finite element solution is in the allowable requirement of the initial conceptual design stage. The approximate expression of the fundamental frequency is deduced in this paper.