Abstract: The sand content and fineness modulus of sea sand are two key indices to the sea sand resource delineation and reserve estimation. Using the two indices is of great significance to the investigation, transfer, exploitation, and supervision of sea sand resources. Based on previous experience in sea sand mining, a system using sand content and fineness modulus to delineate ore for sea sand mining was proposed. Taking a sea sand mining area for backfill in Guangdong as an example, the application of ore delineation with single index (sand content ≥50%) and double indices (sand content ≥50% and fineness modulus≥0.7) 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 very 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.