A resistance supplier for weight training has two switching assemblies, two planetary gear sets, a one-way output bearing, a spindle, and at least one weight. The two switching assemblies respectively control input, output, and fixing of the sun gear, planet carrier, and ring gear in the two planetary gear sets. The output of one of the two planetary gear sets is used as the input of the other planetary gear set. Therefore, the two planetary gear sets provide three reduction ratios and transmit torque to the spindle through the one-way output bearing to drive the weight to rotate. By using reduction ratios of the planetary gear sets to increase the output requirement of the user and switching gears via switching assemblies to change reduction ratios, the resistance supplier can provide sufficient and diverse training resistances with a single weight, thereby reducing the volume.