The trace amount of cell-free DNA (cfDNA) in urine provides a promising diagnosis of urological disease, but its clinical applications are restricted by complicated operations for enrichment and detection. Herein, a portable and visual smartphone-based colorimetric platform is created for urine cfDNA quantification in quick, low-cost, and instrument-free manners. cfDNA in urine is enriched by the zinc ions-dipicolylamino complexes on glass slides and labeled by PicoGreen (PG) to form the DNA-PG complexes with photocatalysis for the generation of a visible output signal. The high sensitivity is related to the amplification of the optical signal of enriched cfDNA through the photocatalysis strategy, which yields greater color changes without interference from other ingredients. Notably, accurate quantitation of cfDNA is achieved with the aid of the smartphone App equipped with collecting and processing Color Grab. As a powerful alternative, such a robust colorimetric biosensor platform is employed for urine cfDNA quantification of clinical samples with excellent linearity compared with the two-stage commercial kits (R > 0.95), helping to distinguish the difference between healthy and urological disease. The comprehensive performance of our developed platform will be the first to enable visual quantification of urine cfDNA in terms of quick detection, low cost, and high portability.