Colloidal carbon dots (C-dots) have attracted a great deal of attention for their unique optical properties. However, it is still a challenge to obtain highly purified C-dots without using multiple-step purification or postsize selection. In this work, a self-precipitation hydrothermal reaction was used to synthesize red-emitting C-dots (R-C-dots) using o-phenylenediamine (o-PDA) as a precursor without using any catalyst. The R-C-dots are able to precipitate on the wall of the reactor, which enables us to obtain solid-state C-dots with high purity. The R-C-dots have a photoluminescence quantum yield (PLQY) of as high as 36.75%, which is among the highest PLQY values reported previously for R-C-dots without using catalysts. The transient PL and transient absorption spectra revealed that 5,14-dihydroquinoxalino[2,3-b]phenazine linked on the surface of the C-dots determined the red luminescence behavior. This work provides a new path for the controllable synthesis of high-purity R-C-dots, showing potential applications in optoelectronic devices.