The study of purely organic room‐temperature phosphorescence (RTP) has drawn increasing attention because of its considerable theoretical research and practical application value. Currently, organic RTP materials with both high efficiency (ΦP > 20%) and a long lifetime (τP > 10 s) in air are still scarce due to the lack of related design guidance. Here, a new strategy to increase the phosphorescence performance of organic materials by integrating the RTP host and RTP guest in one doping system to form a triplet exciplex, is reported. With these materials, the high‐contrast labeling of tumors in living mice and encrypted patterns in thermal printing are both successfully realized by taking advantage of both the long afterglow time (up to 25 min in aqueous media) and high phosphorescence efficiency (43%). A series of simple organic phosphorescence host–guest materials with high performance are successfully developed. With these materials, high‐contrast labeling of tumors in living mice and encrypted patterns in thermal printing are successfully realized by taking advantage of both their long afterglow time (up to 25 min in aqueous media) and high phosphorescence efficiency (43%).