Abstract: To obtain the cleaning methods in high power laser system, the beampath cleanliness experiments and the effects of laser induced damage owing to contamination are studied. The numbers and components of the aerosols are analyzed using the commercial particle counters and SEM. The SiO2 film on K9 is put in the beampath for a certain time, and the transmittance spectra and the morphologies are studied. The laser induced damage threshold of the K9 glass is studied using the 1-on-1 method. The results show that the aerosols keep high concentrations about 7 class after one laser shot with the energy about 5000 J. In this environment, the transmittance of the film descends seriously (about 2.5%), and the aerosols moved onto the surface for clearance in the film. The laser induced damage threshold fluence descends linearly with the cleanliness level of the surface to the maximum decrease of about 10%. The dust and the metal particles are the main contaminations in the beampath, and the source of typical airborne molecular contaminations is discussed and the removal methods are suggested to prolong the life of optics.