Carbon quantum dots (CQDs) are receiving increasing attention for their excellent photostability, low toxicity, and tunable fluorescence performance, but it is still challenging to prepare full-spectrum CQDs using only one single set of precursor and solvent. In this study, P and N co-doped carbon dots (N,P_R-CQDs) with multi-color-emissive tunable fluorescence from blue to green and orange were synthesized from bio-based chitosan, employing phosphoric acid as solvent and dopant. The introduction of phosphoric acid successfully shortens the manufacturing reaction time, and the heteratomic functional groups of P–O/P=O on surface of carbon dots enrich the surface defect states, promoting n–π* transition, and therefore changing their electron structure/energy level to reduce the band gap, thus facilitating emission at long wavelength. Specifically, increasing phosphoric acid concentration leads to characteristic excitation peaks red shift from 320 to 420 nm with the characteristic emission peaks red shift from 406 to 503 nm. A new emission peak at 610 nm appears, enabling N,P_R-CQDs exhibit double emission peaks with fluorescence color from green to yellow to orange. After compositing with carbon nitride (g-C₃N₄), N,P_R-CQDs work as photosensitizer and effectively narrow the energy gap (E_g) of g-C₃N₄ from 2.75 to 2.40 eV, endowing the photocatalyst higher visible light capture ability and broader light response region. The photocatalytic degradation rate of rhodamine B (RhB) under visible light irradiation reaches up to 98.39% within 60 min using N,P_20%-CQDs/g-C₃N₄ as catalyst, which is 1.4 times that of pure g-C₃N₄, and it is demonstrated that ·O²⁻ is the major active substance contributing for RhB degradation by de-ethyl and aromatic ring breakage.