This paper describes a fourth harmonic system for a 1 06 μm laser using nonlinear optic crystals, which are a frequency doubling KTP crystal and a frequency quadrupling LAP crystal. Using computer simulation, we have got optimal parameters of the harmonic device. Some parameters of affecting the harmonic conversion efficiency, phased matching, maximum direction of the effective nonlinear coefficient and threshold power are discussed. This fourth harmonic system offers wide applications to the diagnostics of the soft X-ray ps streak camera and framing camera, nonlinear optics and other ultrafast phenomena study. Finally, a 5 ps ultrashort UV laser has been generated and measured using astreak camera.