High-power single-frequency ultraviolet laser pulses are generated by third-harmonic generation of 1056-nm laser pulses from a Yb-doped fiber laser system. The fiber laser system utilizes a rod-type fiber in its final amplifier for the generation of 108-W 1056-nm laser pulses, and two LBO crystals are used for the efficient second- and third-harmonic generation, respectively. The average output power of 352-nm laser is more than 40 W at a repetition rate of 900 kHz, and the pulse duration is 12 ns with a measured linewidth of less than 150 MHz, which is ideal for the isotope-selective photo-dissociation of formaldehyde molecules.