To address the limitations of conventional polyacrylamide microsphere profile control agents used in low-permeability reservoirs，including poor temperature and salt resistance，and the prominent contradiction between water-absorbing swelling capacity and plugging performance，this study prepared an in-situ self-aggregating nano-profile control agent （In-situ SNP） via emulsion polymerization，using methyl methacrylate and styrene as main raw materials，and N -hydroxymethylacrylamide as the functional monomer. The chemical structure and micromorphology of In-situ SNP were characterized by Fourier transform infrared spectroscopy （FTIR） and transmission electron microscopy （TEM） . Its water-absorbing swelling capacity，salt tolerance，temperature resistance and in-situ self-aggregation behavior were systematically investigated. Furthermore，its injectivity，deep migration performance and profile improvement capacity were evaluated through core flooding experiments. Characterization results revealed that In-situ SNP consisted of spherical particles with a core-shell structure and an average diameter of 60 nm. Performance studies demonstrated its weak water absorption and swelling properties，exhibiting a swelling ratio of only 67 . 18 % even in deionized water at 90 ℃ . It also showed excellent salt and temperature tolerance，dispersing uniformly in brine with Na⁺，Mg²⁺，and Ca²⁺ concentrations of 25 . 0 ×10⁴，5 . 0 ×10⁴，and 5 . 0 ×10⁴ mg/L，respectively. After being aged for 180 days in formation water at 110 ℃，the mass retention rate remained as high as 95 . 66 %. In-situ SNP exhibited favorable injectivity，enabling smooth migration into the deep zones of low-permeability reservoirs. Through condensation reactions between the — CH₂ OH groups in its molecular structure，the agent underwent in-situ self-aggregation to form large-sized aggregates，effectively blocking dominant flow channels and significantly improving the water injection profile. For low-permeability parallel cores with permeability contrasts ranging from 20 . 1 to 40 . 9，the injection of In-situ SNP achieved a profile improvement rate of 86 . 4 % to 91 . 3 %.