In order to solve the problem of ultra-precision optical instruments demanding both low natural frequency and low vibration speed,this paper proposes an ultra-low frequency air flotation vibration isolation method based on complex pendulum and inverted pendulum series.Based on the feature that the horizontal stiffnesses of simple pendulum and inverted pendulum can offset each other,we placed simple pendulum and inverted pendulum in series,and developed a complex pendulum and inverted pendulum se...