Stopcock is a plug with a through hole as the opening and closing pieces, through the plug body and stem rotation to achieve the valve opening and closing action. Plug valve structure is simple, quick opening and closing, fluid resistance. This article made some improvements and calculations of the strength of plug valves used in drilling. Plug the opening and closing member is a perforated cylinder, around the axis of rotation perpendicular to the channel, so as to achieve the purpose of opening and closing channels. Plug valves are mainly used to open and close the pipeline and equipment media purposes. Plug the main advantages are: For regular operation, opening and closing fast, lightweight. Small fluid resistance. Simple structure, relative size, light weight, easy maintenance. Sealing performance is not limited by the installation direction, the flow of media can be arbitrary. No vibration, low noise. However, the plug valves used in drilling often appear after closing the problem is not easy to open, this paper conducted a series of discussions and improvements. Plug valve structure improvements: The lower part of the valve opening the way to make the pressure on both sides of the basic balance, the valve is very easy to open and close, to solve the problems of the original plug valve. Improve the plug valve strength is calculated as follows: 1 shell reference stress calculation A, ball valve hole center section 3-3a, tensile stress concentration analysis A, ball valve section 3-3 a, stretching 2 .. "Chemical Equipment Design Manual "Chapter 5 Pressure Vessel Design, the maximum stress of a cylindrical shell hole is found at the point where the tangential direction of the radial hole edge is parallel to the circumferential direction. The stress concentration factor is 2.5, that is, the stress concentration factor Take 2.5. Yield near the stress concentration point to occur. But less than 2, so stability. b, to reverse the stress analysis of the openings in the pressure vessel design of Chapter 5 of "Chemical Equipment Design Manual", it can be seen that the thin-walled cylindrical shell with the pinholes subjected to the torsion action has the maximum tensile stress appearing on the cross-section of the hole edge and the shaft On the hole by the role of the cylindrical shell of its stress concentration factor of 4. Will not yield. c, the internal pressure p = 105MPa role of the "stress concentration factor manual" found that the cylinder wall open a small hole thick cylinder withstand internal pressure, available stress concentration factor A = 3.03