In order to optimize the determination of safety integrity level ( SIL ) for safety instrumented system ( SIS ) of chemical plant , the deficiencies of existing SIL determination were analyzed .Aiming at the characteristics of shortage and uncertainty in failure data of chemical plant , a fuzzy fault tree analysis&layer of protection analysis ( FFTA-LOPA) model was proposed to calculate SIL of SIS .Taking a low density polyethylene ( LDPE) polymerizing-kettle as example , the explo-sion fault tree of polymerizing-kettle was established .The fuzzy theory was applied to quantitatively analyze the probability of top event , and the SIL of this SIS was determined to be SIL 1.The results showed that through combining two risk analysis theory , the analysis results of this method had better compliance with practical and theoretical statistical results .It has cer-tain accuracy and practicability , and can provide theoretical guidance for quantitative determination of SIL .%为了优化确定化工装置安全仪表系统（SIS）安全完整性等级（SIL），分析了现有确定SIL的不足，针对化工装置的失效数据缺失和不确定性特点，提出模糊事故树－保护层（ FFTA－LOPA）模型计算安全仪表系统SIL。以某低密度聚乙烯反应釜为例，建立了该反应釜爆炸事故树，运用模糊理论定量分析顶上事件发生的概率，最终确定其安全仪表系统安全完整性等级为SIL 1。结果表明：该方法结合两种风险分析理论，分析结果与实际和理论统计结果符合性较好，具有一定地准确性和实用性，可以为定量确定系统SIL提供理论指导。
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