Abstract: A hydrophobic associated polymer nano-silica filtrate reducer FLR-1 with core-shell structure was synthesized using the matrix of block hydrophobic associated polymer P (AM-NaAMPS-MA-b-St) and the filler of nano-silica. The molecule structure, micro-morphological features, and relative molecular weight of FLR-1 were analyzed using FT-IR, SEM, TEM, particle size distribution, and specific surface area analysis combined with GPC experiment. The filtrate reduction property of FLR-1 was further tested via API filtration and HTHP filtration experiments, and the filtration reduction mechanism of FLR-1 in water based mud was finally discussed. The results show that the newly synthesized FLR-1 has excellent filtrate reduction property, with an especially significant effect for HTHP filtrate reduction (HTHP filter loss of 200℃/16 h is 20.5 mL only). Additionally, FLR-1 significantly improves the rheological properties of drilling fluid system and possesses strong saline-alkaline tolerance. At high temperature and high pressure, FLR-1 molecule with special cross-linked structure stabilizes the drilling mud viscosity and keeps the solid particle size distribution reasonably through chemical adsorption, hydration, hydrophobic association, and nano-particle self-assembly. In this way, thin and compact mud cakes are formed, thereby efficiently controlling the HTHP filter loss.