Abstract: This paper mainly studied on the Online Monitoring System for concentration of SF6 gas and its decomposition products in the GIS equipment within the traction substation. The latent mulfunction in the machine could be timely and effectively discovered, and located by detecting the concentration of SF₆ gas and its decomposition products. The study of the System included two aspects, such as hardware design and software design. Hardware device was a portable gas detector, which included photoionization detection module, electrochemical sensors module, multiple sensor modules and so on. Software system defined the operational flow of algorithms between the various modules.