Room-temperature phosphorescence (RTP) carbon dots (CDs) materials have garnered considerable attention from researchers because of their potential applications in anti-counterfeiting. However, the in situ preparation of CDs-based RTP materials with long lifetime and excellent stabilitie remains a considerable challenge. Herein, an in situ manufacturing strategy is proposed to fabricate CDs@Allophane core–shell nanoparticles with double confinement via a simple one-step calcination method. The double confinement arises due to the formation of strong covalent bonds and highly rigid structures via calcination. This double confinement mechanism effectively suppresses the vibration of light-emitting molecules, nonradiative transition of triplet excitons, and environment-induced quenching, affording CDs@Allophane materials with a 1.02 s long lifetime (more than 14 s to the naked eye) and excellent stability. These composites can be preliminarily applied in anti-counterfeiting. This study provides a design approach for the in situ manufacturing of CDs-based RTP materials with a long lifetime and high stability.