In this work, we synthesized ultrabright carbon dots (U-CDs) with photoluminescence quantum yield (PLQY) up to ∼100% using CA and EDA as precursors. When studying their interaction with the Pb²⁺ ion, we found that the PL quenching degree is independent of the U-CDs concentration. This feature provides great convenience for practical detection, which allows the standard curve determination and practical detection to be conducted under different U-CDs concentrations with detection error less than 20%. Based on the experimental observations, a possible mechanism is proposed to explain this phenomenon. To our best knowledge, this work has never been reported before and provides a new idea for the design of novel fluorescent sensors.