Kelly valve also named Kelly cock or drill pipe safety valve. It is manual control valve in drill string circulating system，and it is one of the effective tools to prevent blowout. Kelly valve is divided into Upper Kelly Valve and Lower Kelly Valve，Upper Kelly Valve is used between swivel sub and Kelly, Lower Kelly valve is used between lower end of Kelly and upper end of drill pipe，the special wrench rotate 90 ° switch can achieve on and off according to instruction. In the drilling operation, in order to avoid malignant accidents, shall be connect Kelly valves on the both end of Kelly to prevent loss of drilling fluid and blowout prevention.