Increasing　levels　of　atmospheric　PM2.5　pollution　directly　affect　indoor　environment　quality　and　human　health.　In　order　to　grasp　the　influence　of　atmospheric　PM2.5　concentration　on　indoor　environment,　continuous　measurement　of　indoor　and　outdoor　PM2.5　mass　concentration　were　made　in　naturally　ventilated　near-highway　office　building　from　July　to　August,　2013　in　Beijing.　Based　on　the　measured　data,　we　analyzed　the　variation　rule　of　the　outdoor　PM2.5　concentration　and　indoor-outdoor　(I/O)　PM2.5　mass　concentration　ratios.　The　effects　of　outdoor　wind　speed,　temperature,　relative　humidity　on　the　atmospheric　PM2.5　levels　and　I/O　ratios　were　investigated.　The　study　showed:　1)　under　naturally　ventilation,　indoor　PM2.5　mass　concentration　has　a　significantly　positive　relationship　with　outdoor;　2)　outdoor　wind　speed　and　relative　humidity　are　significantly　correlated　with　outdoor　PM2.5　mass　concentration　and　I/O　ratios,　but　not　outdoor　temperature;　3)　under　the　condition　of　low　wind　speed　and　high　humidity,　it　easily　leads　to　high　outdoor　PM2.5　mass　concentration,　with　low　I/O　ratios.　The　influence　rule　of　atmospheric　pollution　on　indoor　PM2.5　concentration　levels　was　preliminary　studied　under　naturally　ventilation　in　summer.　The　results　can　provide　meaningful　basic　data　reference　for　the　prediction　model　of　the　indoor　PM2.5　concentration　and　effective　removal　of　PM2.5.