Trivalent lanthanide (Ln³⁺)-doped luminescent nanoparticles (NPs) have been extensively investigated as deep-tissue-penetration visual bioimaging agents owing to their exceptional upconversion and near-infrared (NIR) luminescence upon irradiation of NIR light. However, in most cases, the power density of irradiation used for in vivo biological imaging is much higher than that of the reported maximum permissible exposure (MPE) value of NIR light, which inevitably does great damage to the living organisms under study and thus impedes the plausible clinical applications. Herein, by using a facile syringe pump-aided shell epitaxial growth method, we construct for the first time a new class of Ln³⁺-doped KMgF₃:Yb/Er@KMgF₃ core-shell NPs that can be activated by utilizing a 980-nm xenon lamp or diode laser with an ultralow excitation power density down to 0.08 mW cm⁻², a value that is approximately 4 orders of magnitude lower than the MPE value set by the American National Standards Institute (ANSI) for safe bioimaging in vivo. By combining the comparative spectroscopic investigations with atomic-resolved spherical aberration corrected transmission electron microscopy (AC-TEM) characterization, we find that the reduced crystallographic defects are the primary cause underlying such an ultralow-power-excitable feature of the KMgF₃: Yb/Er@KMgF₃ core-shell NPs. And, by the same token, the resultant KMgF₃:Yb/Er@KMgF₃ core-shell NPs also exhibit an anomalous thermo-enhanced photoluminescence (PL) behavior coupled with an excellent photothermal stability that cannot occur in other Ln³⁺-doped core-shell NPs. These findings described here unambiguously pave a new way to prepare high-quality Ln³⁺-doped luminescent NPs with desirable ultralow-power-excitable capability and photothermal stability for future biomedical applications.