According to the magnetoviscous characteristic of magnetic liquid, the viscosity of the magnetic liquid will increase when a magnetic field with its direction perpendicular to the direction of the vortex vector of the magnetic liquid around the magnetic liquid. Using magnetic liquid as the damping fluid of the vibration damper, the damping radio of the vibration system can be changed by adjusting the magnetic field, so that the purpose of vibration damping is achieved. Based on this property, a piston magnetic liquid damper is proposed. The dynamic model of the magnetic liquid in the damper is built according to the flow equation and continuity equation of magnetic liquid. The formula between damping force and vibration speed of the vibration damper and the damping ratio of the vibration damping system are derived. The experiment was designed. A magnetic liquid vibration damper is installed on the free end of the cantilever beam; a coil is used to exert uniform magnetic field on the vibration damper. The effect of the magnetic liquid vibration damper on the damping ratios of the cantilever beam vibration is studied under different coil currents. The experiment results show good agreement between experiment result and theoretical analysis result under the assumed condition. The experiment result also indicates that the damping effect of the piston magnetic liquid damper is enhanced when the current in the coils increases in a certain range.