A fan damping device for adjusting magnetic resistance includes: a support frame, a fan wheel, a ring-shaped magnetic core, a magnetic control unit, and a control unit. The fan blades of the fan wheel can provide air resistance, and the magnetic control unit and the control unit provide adjustable magnetic resistance. By such arrangements, resistance can be increased without increasing the size and weight of the present invention. Besides, the resistance is adjustable as desired to improve convenience of use.