A method for controlling focus loop of an optical storage device includes: moving a lens of an optical pick-up head in a first moving direction; performing a focusing operation when a first S-curve sequence appears in a focus error signal; and when a light beam generated from the optical pick-up head is not focused on an optical disc, performing the focusing operation when a second S-curve sequence appears in the focus error signal; wherein the first S-curve sequence and the second S-curve sequence appear in the focus error signal during one revolution of the optical disc.