According to the technical requirements of sand carrying fluid for reef limestone reservoir to fill ultra light particles in cracks. A sand carrying fluid system was studied with filtered seawater as the base fluid and drainage aid and dispersion agent as the main agent. When the surface tension of the sand carrying liquid system is 24.31 mN/m and the interfacial tension is 0.122 mN/m, it has a better assisted drainage ability, and the ultra?light particles are well dispersed in the sand?carrying liquid. Compared with ordinary ceramsite, the settling velocity is reduced from 0.58 cm/s to 0.35 cm/s, it has a good suspended sand effect on ultra light particles. The ultra light particles with 10%~50% sand ratio can be effectively suspended under stirring and settled within 270~470 s after standing. The sand carrying fluid system can meet the needs of carrying and filling ultra light particles in cracks.