Provided is a resistance device of a fitness machine comprising a wind drag unit and a magnetic resistance unit. The wind drag unit includes an air volume adjusting disc. The magnetic resistance unit includes a guide rail disc, a metal ring sheet, at least one magnetic seat, and at least one guide post. The guide rail disc has a guide disc rail through which the guide post penetrates. The magnetic seat is used to generate magnetic resistance with the metal ring sheet, and the magnetic seat has a magnetic seat rail. The guide post is connected to the air volume adjusting disc, penetrates through the guide disc rail, and extends into the magnetic seat rail. The guide post moves in the magnetic seat rail with rotation of the air volume adjusting disc to drive the magnetic seat to pivotally rotate relative to the guide rail disc.