An optical compensation system for a laser beam and an excimer laser annealing device are provided in the present disclosure. The optical compensation system for the laser beam includes a laser source, a beam splitter and a reversion assembly. The laser beam emitted by the laser source enters the beam splitter, and is divided by the beam splitter into a first light beam and a second light beam having different transmission paths. The second light beam is reversed and reflected by the reversion assembly, enters the beam splitter, and exits from the beam splitter together with the first light beam. An asymmetry of the second light beam reversed by the reversion assembly is different from an asymmetry of the first light beam.