A magnetic resistor control assembly for a body training machine comprises a main shaft; a rotor, a stator, an adjusting unit, a sliding block set and a controller. An inner disk and an outer disk clamp of the stator clamping the adjusting unit. The sliding block set has guide tracks, an outer lateral wall of each guide track having a track trench. A lower middle portion of the sliding block set has a round hole; the roller being installed and rollable in the guide track; and the retaining rod of the roller is placed within the track trench. A controller is installed at an outer side of the stator; the controller having a transfer pin which displace in the straight grooves. When the transfer pin drives the sliding block set to move in the straight grooves, the gap between permanent magnets and the magnetic isolation ring is changed.