According　to　PM2.5　contamination　in　residential　building,　a　mathematical　model　is　established　about　the　effect　of　natural　ventilation　on　PM2.5　pollution　control　based　on　calculus　method.　On　the　basis　of　some　actual　cases,　such　as　indoor　smoking　and　cleaning,　indoor　PM2.5　concentration　is　calculated　for　different　air　change　rate　and　outdoor　air　quality.　The　concept　of　critical　ventilation　rate　is　provided.　The　conclusion　of　the　paper　provides　some　advice　on　natural　ventilation　and　indoor　life　behavior.