A near‐infrared (NIR) induced decomposable polymer nanocapsule is demonstrated. The nanocapsules are fabricated based on layer‐by‐layer co‐assembly of azobenzene functionalized polymers and up/downconversion nanoparticles (U/DCNPs). When the nanocapsules are exposed to 980 nm light, ultraviolet/visible photons emitted by the U/DCNPs can trigger the photoisomerization of azobenzene groups in the framework. The nanocapsules could decompose from large‐sized nanocapsule to small U/DCNPs. Owing to their optimized original size (ca. 180 nm), the nanocapsules can effectively avoid biological barriers, provide a long blood circulation (ca. 5 h, half‐life time) and achieve four‐fold tumor accumulation. It can fast eliminate from tumor within one hour and release the loaded drugs for chemotherapy after NIR‐induced dissociation from initial 180 nm capsules to small 20 nm U/DCNPs.