To study the dynamic characteristics of long-span asymmetric suspension bridges，the three-dimensional finite element model of a long-span suspension bridge with unequal height support of main cable is established by ANSYS. The asymmetric parameters of the structure are taken into account when the frequency of the vibration is calculated, and the first 20 modal modal analysis is carried out, and the effects of the key structural parameters, including the ride-span ratio, the asymmetric structural parameter, the stiffness of the main girder, and the stiffness of the main tower, on the vibration frequency are analyzed. The results indicate that the sensitivity of different parameters to the vibration fundamental frequency of asymmetrical suspension bridges is different. The first vertical and torsional vibration frequencies decrease with the increase of the rise-span ratio, and the anti-symmetric frequency is more sensitive to the rise-span ratio than the symmetric vibration frequency. The first anti-symmetric vertical and torsional fundamental frequencies of asymmetric suspension bridges are not affected by asymmetric structural parameters, while symmetrical vertical and torsional fundamental frequencies decrease with the increase of asymmetric structural parameters. The first transverse vibration frequency with the stiffness of main girder is very sensitive. The original mode of structural vibration has changed when the stiffness of main girder is increased to 3 times the original bridge. However, the change of the bending stiffness parameters of the main tower has little effect on the structural frequency. The results can provide reference for the structural design and dynamic analysis of long-span asymmetric suspension bridges.