We develop a power saving frequency tuning and locking system of a deep ultraviolet (UV) laser at 253.7 nm, which will play an important role in a magneto-optical trap for neutral mercury atoms. Combined with two cascade acousto-optical modulators (AOMs) and frequency modulation (FM) spectroscopy in a mercury vapour cell, the deep UV laser can be locked and tuned on 1S0-3P1 transition of Hg atoms, and the power for laser cooling can be saved. FM spectroscopy is also observed by demodulating the saturated absorption spectroscopy (SAS) of the FM laser. The UV laser is well locked on 1S0-3P1 transition of 200Hg atoms with the instability less than 0.1 MHz.