Abstract: With the development of frequency stabilization technology in recent years, the linewidth of the 243nm laser based on a frequency-quadrupled external cavity 972nm diode laser has been compressed to sub-Hz level by locking to an ultrastable Fabry-Perot cavity. This has provided a new tool in high-resolution spectroscopy for studying the 1S-2S transition in atomic hydrogen. Recent progress in the development of an ultrastable 243nm laser and its applications in precision hydrogen spectroscopy will be reviewed, as well as some key issues of Fabry-Perot cavity manufacture.