Based on Maxwell^, s eddy current mirror image theory, the resistance expression of magnet moving at low speed above an infinite thin conductor plate is quantitatively calculated. It is found that the resistance is proportional to the moving speed, the square of magnetic moment, the thickness of conductor plate, and decreases with the fourth power of the distance between magnet and conductor plate. The magnetic pendulum-aluminum foil system is constructed by improving pendulum experiment with other variables keeping unchanged. The minimum distance between the magnet and the aluminum foil is changed for several times. The results obtained from the theory and the experiment are in good agreement.