A assessment model which based on risk matrix is proposed in this paper, for improving objectivity of the air security assessment on big enclosure space such as defense construction. Risk factors of the enclosure space are counted quantitative by two dimension matrix. The weight of the risk factor is got by Borda and AHP (analytical hierarchy process). The risky numerical value is reckoned by the risk matrix and a storehouse is taken for an example. It proves that the assessment model can quantify qualitative process and reduce factitious loss in a large part.