Copper ion (Cu²⁺) plays an important role in the human body because it is beneficial for metabolism. However, an excessive or slight amount of Cu²⁺ can cause various symptoms. Therefore, it is necessary for human health to realize the trace and visual detection of Cu²⁺. Referring to traditional fluorescence test papers, the qualitative and semi-quantitative detection of Cu²⁺ could be realized by a dual-carbon dots (CDs) ratiometric fluorescent paper-based sensor with the advantages of environmental protection, portability and low cost. In this paper, the inkjet-printed test paper with the best mixing ratio of the two CDs has been researched to maximize the spectral energy transfer of ion detection (maximum color gamut expansion). Among them, the preparation method of b-CDs has been improved, increasing the photoluminescence quantum yield (PLQY) to 88.9%. The sensitivity detection limit of the double emission ratio sensor was 0.15 nM in solution, and the human eye can distinguish at least 3 μmol L⁻¹ Cu²⁺ in the paper-based sensor. Compared with the traditional single-emission sensor, the human eye was more sensitive to the color change of the emission ratio sensor. The results indicate that we not only realized the micro detection of Cu²⁺ with convenient methods, but also provided a promising strategy for the visual detection of Cu²⁺.