A casing is used for being rotatably disposed in a plasma jet system. The casing is rotated around a central axis. The casing comprises a main body and a plasma nozzle. The main body has a first cavity. The plasma nozzle is disposed under the main body and has a second cavity and a straight channel. The second cavity is connected to the first cavity. The straight channel is located at a side of the plasma nozzle opposite to the main body and connected to the second cavity. The straight channel has an extension axis which is substantially parallel with the central axis and separated from the central axis by an interval. Plasma generated by the plasma jet system jets out through the straight channel.