Ram blowout preventer is an important well control equipment in oilfield production and an important guarantee for wellhead safety control. The internal structure of blowout preventer is complex. To guarantee the safe operation of the equipment, and provide basis for structure optimization, it is necessary to analyze and calculate the strength of the pressure bearing structures. Based on JB 4732—1995 Steel pressure vessels——design by analysis and the finite element method, the stress classification method and the ultimate load method were used to analyze the strength of the shell. The sub modelling technique technology was used to analyze and optimize the local section in detail. The results show that: 1)the strength of the shell meets the standard requirements under various working conditions and there is still a certain margin; 2) by creating fillet on the edge of structure with sub-modelling technique and conducting detailed research on critical local part of the structure, the full analysis of overall and local stress is conducted. Together with an improved calculation efficiency, the stress intensity of local film stress strength SⅡ and film and bending stress strength SⅢ decreases by 27%and 36%respectively, as compared to earlier calculation. The research results can provide some reference for strength analysis and optimization for this kind of equipment.