Abstract: Applying both FMECA (Failure Mode Effects and Criticality Analysis) and FTA (Fault Tree Analysis) to demonstrate the feasibility of the proposed combination scheme, the system level and device level of the failure mode and the degree of hazard for the IBOP(internal blowout preventer) combination scheme of nitrogen drilling is analyzed. Then, the common failure modes of each device are found out and the improvement measures put forward. Lastly, the criticality of the failure modes and that of each IBOP group in the system are quantitatively calculated so as to determine the top event of FTA. Through the FTA, all the bottom events and the minimal cut sets which lead to the main failure of IBOP group are found out, and then the structure importance of the bottom events and the influence of the bottom events combinations determined. The prior improvement measures are firstly taken to deal with the important bottom events and combinations of the basic events which have a heavy influence on the system′s failure. Then the reliability of the weakest parts is calculated, which can derive more focus on the design improvements and fault monitoring of the weakest parts. With the researches on the reliability of the IBOP groups of nitrogen drilling, the main failure modes which lead to the system′s failure are obtained, which include the cone seal failure of arrowtype check valve, the reset spring failure of flap valve, the crushed valve ball of input type check valve, the fractured buckle of the bypass valve and the locked rotary ball of plug valve. Then all reasons that lead to the aforesaid five failure modes are analyzed and the corresponding improvement put forward.