The present invention discloses a sheet feed scanner including an auto document feeder, a flatbed scanner and a power-switching mechanism. The power-switching mechanism includes a first shaft, a power source, a clutch, a first gear, a second gear, and an elastic limitation structure. The clutch includes a first incline and a second incline. The elastic limitation structure pushes the first incline or the second incline of the clutch. When the first shaft is rotated in a first direction, the first incline is pushed by the elastic limitation structure so that the clutch is coupled to the first gear for driving the automatic document feeder. When the first shaft is rotated in a second direction, the second incline is pushed by the elastic limitation structure so that the clutch is coupled to the second gear for driving the flatbed scanner.