Abstract: The ore delineation index of marine sand is the core content of marine sand resource estimation, which is of great significance to the investigation, exploitation and supervision of marine sand resources. Based on extensive pratical experience in marine sand investigation, a system using sand content and fineness modulus to delineate ore for marine sand investigation was proposed. Taking a marine sand mining area for backfill in Guangdong as an example, the application of ore delineation with single-index (sand content ≥50%) and double-index (sand content ≥50% and fineness modulus≥0.7) in delineating ore was compared. Results show that using the single-index method could easily result in overestimation of reserve. However, using double-index method requires the sand content to meet the standard, and ensures the coarseness of sand to be up to standard, which could effectively avoid excessive amounts of extremely fine-grained sand from being delineated as ore, thus ensuring the mining value of the delineated ore bodies. Therefore, double-index method is more scientific and practical.