A footrest extension device for a chair includes eight connecting rods: a first connecting rod is connected to a main frame, and pivotally connected to a second connecting rod and a third connecting rod, respectively; the second connecting rod is connected to the main frame, and pivotally connected to a fourth connecting rod; the third connecting rod is pivotally connected to the fourth connecting rod, a sixth connecting rod and a eighth connecting rod, respectively; a fifth connecting rod is pivotally connected to the fourth connecting rod and the sixth connecting rod, respectively; the sixth connecting rod is pivotally connected to a seventh connecting rod; the seventh connecting rod is pivotally connected to the eighth connecting rod; and the second connecting rod, driven by a driving device, drives the other connecting rods to move, so the footrest extension device is folded or extended.