248 nm KrF excimer laser is the preferred choice for writing fiber Bragg grating (FBG). A miniaturized KrF excimer laser is developed according to the requirements of FBG writing, and the mechanical structure design and improvement are also carried out. The output laser energy and efficiency dependent on the charging voltage, discharge capacitance, components of mixed gas and gas pressure in storage cavity are investigated experimentally. The laser’s performance is optimized. The output repetition rate reaches 200 Hz, and the single-pulse output energy is up to 16 mJ. The energy instability is less than 2%, and the near-field spot dimensions are 6 mm×3 mm near the output window. The maximal output efficiency is 1.24%. The experiment to write FBG by using phase-mask method is carried out using the laser with good results.