Abstract: By means of building transmitting and focusing optical system, the beam quality of high power ultraviolet ultrashort laser pulse system was researched by using a CCD camera. Relevant parameters obtained from the measurement are the spatial intensity distribution, the size of focal spot and beam quality β factor of the ultraviolet seed laser and the amplified pulse after its amplification by a KrF laser system. The result shows that the beam quality of the ultraviolet seed laser beam is good. β factor is closed to 1. After amplification the beam quality worsens and β factor increases to about 6. Through analysis, surface defects of optical elements may be the major factor which leads to the degradation of the laser beam quality.