Study on characters of suspension bridges free vibration is basic to analyze wind resistance, shock resistance, and other dynamic behaviors. Current studies are usually focused on earth-anchored suspension bridges. Compared with earth-anchored suspension bridges, self-anchored suspension bridges are anchored on the girder end which makes girder under the compression condition, cable longitudinal displacement can not be neglected, in addition, large ratio of rise to span leads to cable large tilt angle. Based on functional analysis method, it uses variation calculus to derive differential equation of two-tower and three-span continuous self-anchored suspension bridges free vibrations, taking into account cable tilt angle and cable longitudinal displacement. According to the simplified differential equation, it draws to a formula that can calculate self-anchored longitudinal and vertical vibration frequencies. Finally, two examples are carried out to check the formula’s reasonableness.