Continuously tunable UV radiation is obtained by oollinear doubling and non-collinear mixing from visible radiation of pulsed dye laser. A closed loop method, in which a sample light spot is separated into two equipower parts in the direction perpedicular to phase matching at beginning wavelength of tuning range, then the powers of two parts are compared and balanced in the tuning is reported. This method is analyzed theoretically and demonstrated successfully in our experiment. The auto-tracling efficiency is better than 90%.