Amphiphilic nanoreactors have been recently used to fabricate photoluminescent carbon-rich polymer nanodots (PCPNs). However, the applications of PCPNs have been limited by their requirements for high temperature and toxic organic solvents or catalysts and the difficult control of their luminescent properties. Herein, a novel and facile strategy is reported for the synthesis of controllable PCPNs. This strategy involves the use of in situ vesicular nanoreactors under mild photoirradiation with fatty acid as the precursor. The conjugation degree of the uniformly sized PCPNs can be increased by extending photoreaction time, thus enabling the tuning of the optical properties of PCPNs. The PCPNs, which feature controllable and outstanding luminescent properties, low cytotoxicity, and biocompatibility, are successfully applied in bioimaging and as fluorescent ink. The present strategy is an attractive and facile platform for the preparation of carbon-rich nanomaterials with controllable photoluminescence.