As a novel protein knockdown tool, proteolysis targeting chimeras (PROTACs) can induce potent degradation of target proteins by hijacking E3 ubiquitin ligases. However, the uncontrollable protein disruption of PROTACs is prone to cause "off-target" toxicity after systemic administration. Herein, we designed a photocaged-PROTAC (phoBET1) and loaded it in UCNPs-based mesoporous silica nanoparticles (UMSNs) to construct a NIR light-activatable PROTAC nanocage (UMSNs@phoBET1) for controllable target protein degradation. Upon NIR light (980 nm) irradiation, UMSNs@phoBET1 nanocages could be activated to release active PROTAC via a controlled pattern for degrading bromodomain-containing protein 4 (BRD4) and inducing MV-4-11 cancer cell apoptosis. In vivo experiments demonstrated that UMSNs@phoBET1 nanocages were capable of responding to NIR light in tumor tissues to achieve BRD4 degradation and effectively suppress tumor growth. This NIR light-activatable PROTAC nanoplatform compensates for the current shortcomings of short-wavelength light-controlled PROTACs and presents a paradigm for the precise regulation of PROTACs in living tissues.