A ferrofluid damper with simple structure is proposed. Dynamic model of the flow of the ferrofluid in the damper is built according to Newton’s second law and the continuity equation. Combining this dynamic model with the oscillating energy of a elastic cantilever beam, the logarithmic decay rates of the beam installed with the damper can be obtained, which are also measured experimentally using different dampers composed of magnets with different radius, different gaps between the shells and the magnets or ferrofluid with different saturation magnetization. The experimental results have good agreement with the predictions by the theory. We can also find that the damping effect enhances when the radius of the magnets increases and there is an optimal gap between the shell and the magnet, using which the vibrating beam can attain its maximal logarithmic decay rate.