A simple, instrument-free, paper-based analytical device with dual-emission carbon dots (CDs) (blue CDs and red CDs) was developed for the semiquantitative, visual, and sensitive speciation analysis of lead ions in a real sample with a sensitive detection limit of 2.89 nM. When a paper strip was immersed into the sample solution, the blue fluorescence was quenched by Pb2+ in solution, while the red fluorescence served as a background reference without color change, and significant color evolutions from blue to red were observed under the ultraviolet lamp, resulting in a semiquantitative visual detection. Furthermore, a smartphone was used in the visual detection of lead ions by identifying the RGB value of the fluorescent probe solution and corresponding paper strip. The application of smartphones and fluorescent paper strips has greatly shortened the detection time and reduced the cost of detection, providing a new strategy for the on-site and semiquantitative detection of heavy-metal ions in water samples.