The present invention relates to a folding unit of a motorized treadmill which includes a main body, a deck, a deck transmission assembly and a folding mechanism. Mainly, an auxiliary folding assembly is pivotably connected to the slide shaft, and a movable support fixed at the rear end of the auxiliary folding assembly 50 is able to make a to-and-fro sliding movement on the main body with the slide shaft so that the movable support of the auxiliary folding assembly is permanently situated behind the center of gravity of the deck 20 when the deck folds up.