Continuous-wave tunable deep ultraviolet (UV) laser is commonly realized by extra-cavity frequency doubling of near infrared ray (NIR) or visible laser, but the bandwidth of cavity locking circuits limits the tunability of laser. A feed-forward control method is applied in the frequency quadrupled UV laser, which is used for laser cooling of neutral mercury atom; that is to say, synchronous adjust ment two series frequency doubling cavity when tuning the frequency of fundamental laser. By this method, not only the power noise is induced by laser frequency tuning reduced, but also the frequency tuning range of the UV laser is enhanced. Hence, the tunability of UV laser is considerably increased. When the laser frequency on the Doppler-free dichroic locking spectroscopy of neutral mercury atom is locked, it is found that the laser frequency noise can be reduced by the application of feed-forward control circuit. The enhancement of the tunability and stability of UV laser provides a lot of advantages in the experiments of laser spectroscopy and laser cooling of mercury atom.