Multi-peak emission is one of the most attractive properties of carbon dots (CDs). However, the mechanism of multi-peak emission is still not clearly explained, especially for biomass-derived CDs. Herein, we use cyanobacteria as carbon source, and prepare blue-light CDs and red-light CDs with triple emission via a facile microwave hydrothermal route. The CDs maintain stable and efficient fluorescence in solid state (QYs= 41.76%, 14.03%) with narrow half-peak widths of 74 and 30 nm. The experiment greatly improves the synthesis yield of CDs (40%), and expanding the experimental scale fivefold to achieve the gram-scale production. In-depth mechanism analysis revealed that the introduction of alkali can increase the self-doping content of N in CDs and convert pyrrolic N into graphitic N, further forming a hydroxyl shell on the surface of CDs. The synergistic effect of hydroxyl groups and graphitic N could adjust the energy gap, and triple-peaked emitting R-CDs are obtained. R-CDs are successfully applied to the visual ratiometric fluorescence sensor for Pb²⁺, which has a large linear range of 0-50 μM, 100-300 μM and a low lower detection limit of 0.85 μM. And R-CDs show a significant linear enhancement from pH 7 to 12, indicating R-CDs potential in the sensor field.