The effect of KrF pulsed excimer laser irradiation on intrinsic defects，ultra-violet (UV) emission and surface morphology of ZnO thin films was investigated，and also the origin of room temperature UV emission was discussed in detail. It was found that，the KrF laser can break the Zn—O bonds; therefore，the concentration of VO (or Zni) defects increases，leading to the decrease of resistivity and increase of carrier concentration. By adjusting the laser energy densities，the donor defect concentration can be controlled in a wide range. Simultaneously，under the heat of laser，the melting grains connect with each other，resulting in the great decrease of surface roughness. Room temperature UV emission of ZnO film is composed of contribution from free-exciton (FX) recombination and its longitudinal-optical phonon replica (FX-LO), the defect density determines the relative strengths of FX to FX-LO emission intensities，which strongly affect the peak position and intensity of UV emission of ZnO film. This investigation indicates that the laser irradiation is an effective technique to modulate the exciton emission by controlling the defect density，which is important for the application of high performance of UV emitting optoelectronic devices.