The invention discloses a magneto-rheological vibration isolator for a precision machine tool, comprising a piston rod, a rubber main spring and an upper belleville spring, wherein the piston rod is penetrated in the rubber main spring along the vertical direction; the piston rod is sleeved by the upper belleville spring which is located above the rubber main spring and is connected in parallel with the rubber main spring; and the piston rod is also provided with a regulating structure for regulating the upper belleville spring to be in a negative stiffness state. According to the magneto-rheological vibration isolator, the upper belleville spring connected in parallel with the rubber main spring is arranged on the basis of the existing rubber main spring, the upper belleville spring is enabled to generate predeformation to obtain the required negative stiffness through the regulating structure during use, in this way, the motion stiffness of the vibration isolator can be reduced on the premise that the vibration isolator is kept to have sufficient support stiffness, the static and dynamic stiffness uncoupling of a supporting structure can be realized, and the inherent frequency of a vibration isolation system can be effectively reduced. Due to the adoption of reasonable design, the motion stiffness at an equilibrium position of the vibration isolation system is enabled to be close to zero, thereby realizing ultralow-frequency vibration isolation.