A lower limb stretching workout device includes a frame unit, two sliding plates, a motor assembly, and two cranks. The frame unit has two guide rails opposing each other. Rollers are disposed at the sliding plates and slidingly movable along the guide rails. The cranks each have a driving end and an acting end. The driving end is pivotally connected to a cam. The acting end is pivotally connected to a lug beneath each of the sliding plates. The motor assembly drives a rotating shaft to rotate through a transmission module. The torque of the rotating shaft is converted by the cams into a push/pull for driving the cranks to push/pull the sliding plates, thereby causing the sliding plates to slidingly move toward each other and away from each other. A user's two feet rest on the sliding plates, respectively, to stretch the user's lower limbs by extension and flexion.