The forces applied on the individual particle of drilling cuttings are analyzed and the principle of hydro cyclone is discussed. It is pointed out that the column of whirlpool gaseity in the center has a marked effect on mud purification. The improved design of hydro cyclone, whose structure parameters can be adjusted either manually or automatically, is brought forward. The improved desander proved to be effective through the experiment of critical granularity, minimum mud exhaustion and of preventing sand discharged hole from being jammed. The device can be used with advanced drilling equipment. The application value of this device will be more outstanding, especially in the west area scanty of water.