Though statistically analyzing ten typical wells of kelly cock valve failure in certain oilfields, the main failure modes of the kelly cock valve were determined. Aiming at these modes and combining with mechanical analysis of knob, upper seat and main body of the kelly cock valve, its finite element calculation had been implemented and the weakness of all parts was found. By numerical modeling of the flow field of a kelly cock valve it was found that the velocity of fluid rapidly increased at upper and lower valve seats and this could easily prick out ball valves and valve seats, there was a annular low-velocity zone of minimum speed between the ball valve and the inner wall of the main body and this caused solid aggregation at this area. The research in this paper made the modes and causes of kelly failure to be recognized systematically. The results provided theoretical foundations of further improving the design and reasonable using of a kelly cock valve.