Herein, an innovative coreactant modifier-based amplified ECL sensing method was developed to provide a convenient and effective amplified ECL emission. Firstly, nitrogen-rich quantum dots (N-dots) were prepared. N-dots have much nitrogen functional groups, which provide more reactive sites on the surface in the ECL reaction. Furthermore, we investigated the action mechanism of H2O2 as coreactant modifier to promote the generation of more oxidant of coreactant in the N-dot/K2S2O8 coreactant ECL pathway. The coreactant modifier can generate ca. 40- and 7.7-times intensity over that of H2O2 or K2S2O8 as an individual coreactant, respectively. Finally, the proposed biosensor was designed to accurately quantify galactose (GA) from 0.5 μmol L-1 to 15 mmol L-1 with a limit of detection (LOD) of 0.16 μmol L-1. This coreactant modifier-based amplified ECL sensing method significantly improved the sensitivity and accuracy of enzyme sensors. And it was applied in spiked human serum samples with satisfactory results. Moreover, the change of the ECL signal can be easily imaged and observed with the help of a smartphone camera, which satisfied the needs of point-of-care test of GA.