We report an experimental demonstration of polarization smoothing by using a specially cut KDP plate in the convergent ultraviolet (351 nm wavelength) section of a high-power laser system. A 410 mm×410 mm×10 mm sized KDP plate placed after the final focus lens successfully reduced the focal spot intensity contrast to 75.6%, which is in good agreement with the theoretical expectation. In our experiment, no obvious transverse stimulated scattering was observed with the maximum 351 nm laser energy of 4032 J/3 ns, thus indicating the operation safety margin of our laser system.