Abstract: The present disclosure provides an excimer laser system. A master oscillator chamber may generate laser pulses with a narrowed line width and a small energy by means of a line width narrowing module, as a seed light. The seed light is refracted by a master oscillator wavefront engineering box and then incident into a power amplifier chamber through a beam splitting system. The beam splitting system, a first high reflectance mirror, a second high reflectance mirror and a third high reflectance mirror may constitute a quadrilateral annular optical path, The power amplifier chamber may have a first pair of Brewster windows and a second pair of Brewster windows, wherein the first pair of Brewster windows is located in a first optical path of the annular optical path along with a discharging electrode of the power amplifier chamber, and the second pair of Brewster windows is located in a second optical path of annular optical path which is parallel to a first amplification optical path.