There are two different sides of ambiguousness and in distinguishability among systematic safety.  Fuzzy synthetic assessment can only solve ambiguousness problem from front, but Vague sets have fairly strong and flexible intuition expression ability for handling indefinite data. From this, this paper suggests that Vague sets be used in the synthetic assessment, then determines the weight of each assessment index with improved AHP, and establishes the synthetic assessment model for systematic safety based on Vague sets. Through the assessment for a certain high building fire hazard in Shanxi Province, it proves that this model can make the assessment result of systematic safety more accurate and reasonable.