A novel ultraviolet (UV) femtosecond laser source is experimentally studied based on the large-mode-area photonic crystal fiber femtosecond laser system. Walk-off effecting due to the group velocities mismatch between the fundamental wave and the second harmonic is analyzed theoretically. The average power and the conversion efficiency of second harmonic generation with different crystal length are compared experimentally. Second harmonic generation (SHG) and fourth harmonic generation (FHG) of 1040 nm fundamental wave are demonstrated,and high power UV femtosecond laser is obtained out of cavity at the repetition rate of 50 MHz by using two pieces of Type-Ⅰ BBO crystal,length of which is 5 mm and 0.18 mm respectively. The fundamental wave is generated from the photonic crystal fiber amplifier with pulses duration of 110 fs. At the average power of 20 W,8.88 W frequency doubled green laser wave is obtained with the conversion efficiency up to 44.4%,and 656 mW FHG UV laser is also achieved with a single pulse energy of 13 nJ. The conversion efficiency from the second harmonic (SH) to the fourth harmonic (FH) is up to 7.39% at the maximum power.