Abstract: In order to satisfy the actual need of accurate measurement of SF₆ gas concentration in the electrical system, a portable SF₆ gas sensor was designed based on non-dispersive infrared (NDIR)differential detection principle, which adopted the single-beam and dual-wavelength optical structure. In order to improve the compactness and sensitivity of the system, the optical simulations of the straight and trapezoidal reflective gas chambers were carried out, and the gas chamber type was finally determined. As for designing hardware circuit, a small signal amplification filter circuit was designed to effectively extract the useful signal from the noise. The sensor uses the high precision and performance analog-to-digital converters to convert analog electrical signals into digital signals into single chip processor, which greatly improves the detection accuracy. The experimental results show the sensor can accurately measure the SF₆ concentration in a range of 0~2×10^-3, with the full range accuracy up to 4.2%.