Abstract: The key technology about the design of 90 t Ultra High Frequency Arc Furnace's dedusting sys-tem, technical flow, and automation control was introduced. And the material of dedusting filter bag, cleaning dust technics and the temperature of the gas were analyzed. The dedusting system of the 90 t UHP arc furnace in Hengyang Steel-making Co. was run into use for six months. It worked well and stable. When the arc furnace worked normally, it could work automatically. It was more advantage than the first steel making factory. In fact, because of the use of the dedusting system, it improved the pollution to the environment and improved the pro-ductivity, so good economy and social benefit were got.