Rheological modifier was a key additive to enable flat rheology property for water-based drilling fluids. A specific rheological modifier was synthesized by reverse microemulsion method using acrylic acid （AA），acrylamide （AM） and 2-acrylamido-2-methyl-propane sulfonic acid （AMPS） as raw materials. The structure，morphology and thermal stability of synthesized product were characterized. Meanwhile，the rheological behavior of rheological modifier and traditional viscosifier in water-based drilling fluid was comparatively investigated. Also，the flat-rheology mechanism of water-based drilling fluid was explored，in combination with the molecular aggregation structure of rheological modifier. The results showed that the average molecular weight of rheological modifier was 7.5×10⁶，the average particle size was 258.7 nm，and the temperature resistance was high up to 260℃ . In water-based drilling fluid，the rheological modifier adjusted the rheological property well，which could dramatically reduce the influence of temperature variation （4—65℃） on rheological parameters. It was concluded that the core-shell structure of rheological modifier should reinforce molecular rigidity and stability. In particular，a layer distribution of hydrophilic radicals around the shell could compensate radical loss caused by harsh environment，and sustain the total quantity of effective radicals. As a result，the association action between polymeric molecular and clay layers could be well maintained，and the flat-rheology of water-based drilling fluid could be realized.