This paper presents the first-time results on design and implementation of a fiber Bragg grating (FBG)-based distributed temperature sensor up to 500°C in a high-voltage and high-electromagnetic interference (EMI) environment. The multiple FBGs at different peak wavelengths were inscribed on a single fiber of length 2 m at the spacing of about 30 cm by controlling the geometrical divergence of a highly coherent UV (255 nm) beam falling on a biprism. The developed multipoint sensor is tested on a high-voltage and high-EMI environment such as a copper bromide (CuBr) laser. The temperature of 200°C to 500°C is monitored online along the operating laser tube length.