In order to improve the safety performance of well control semi sealed ram blowout preventer（BOP）, failure analysis and sealing performance evaluation were carried out for the semi sealed ram blowout preventer（BOP）in service, and the characterization test of ram body and material tensile and impact tests were completed. Based on the stress analysis of semi sealed ram during overflow shut in, and the stress and strain test data of ram body material, a full size semi sealed ram blowout preventer（BOP）model was established. The mechanical properties of ram and sealing rubber core were simulated under the combined load of downhole high-pressure fluid and cylinder pressure. The results show that when the cylinder pressure exceeds 9.4MPa , the groove root of the ram body just enters the yield deformation, and the maximum equivalent stress occurs at the groove root of the ram body, leading to premature fracture failure of the ram body. The analysis also found that the stress concentration in the contact area between the rubber core and the upper end of the ram was the most obvious. It is concluded that the upper part of the rubber core is squeezed due to the combined load, thus losing the sealing ability. Finally, measures to reduce ram stress concentration are proposed, which provide guidance for safe operation of semi sealed ram blowout preventer（BOP）under overflow.