The invention discloses an active-passive combined vibration isolator based on positive-stiffness and negative-stiffness parallel connection. The active-passive combined vibration isolator based on positive-stiffness and negative-stiffness parallel connection comprises a basic platform (10), a load-bearing platform (30), a passive vibration isolation unit and an active vibration isolation unit, wherein the passive vibration isolation unit is provided with a positive-stiffness air spring (20) and a magnetic negative-stiffness mechanism (28) with adjustable negative stiffness in the first direction, so that the inherent frequency in the first direction is decreased, and the passive vibration isolation unit is provided with a stiffness-adjustable positive-stiffness leaf spring and a negative-stiffness inverted pendulum in the second direction, so that the inherent frequency in the second direction is decreased; the active vibration isolation unit is arranged at the outer side of the passive vibration isolation unit, the upper end of the active vibration isolation unit is linked with the load-bearing platform (30), and the lower end of the active vibration isolation unit is linked with the basic platform (10); and the active vibration isolation unit comprises displacement sensors, speed sensors, a proportional pressure valve and a voice coil motor, and the voice coil motor is used for fine adjustment of the load-bearing platform, so that vibration is further isolated. The active-passive combined vibration isolator based on positive-stiffness and negative-stiffness parallel connection has high bearing capacity, and active and passive vibration isolation of two degrees of freedom is achieved at the same time.