In the drilling process of deep-water oil and gas fields，due to the low temperature near the bottom mud line，it is easy to appear thickening，plugging and slurry running of drilling fluid. In order to solve the problem，the thermosensitive thickening copolymer（ASSN）was prepared from acrylamide，N-vinyl caprolactam，divinylbenzene and sodium allyl sulfonate. Then the flow pattern regulator with constant rheological properties at low temperature was prepared by mixing ASSN with fenugreek gum. The solid-free constant rheological drilling fluid with reservoir protection for deep-water drilling was prepared by mixing flow pattern regulator with other treatment agents. The performance of ASSN，flow pattern regulator and drilling fluid was evaluated. The results showed that the critical association temperature of ASSN solution was 27 ℃. When 0.5% ASSN and 0.3% fenugreek gum were used as flow pattern regulators，the viscosity of the aqueous solution was stable in the range of 4—60 ℃ . The flow pattern regulator played a good role in regulating the rheology of drilling fluid at low temperature. The weighting agent had little effect on the control ability of drilling fluid at low temperature. The flow pattern regulator had good compatibility with other treating agents of drilling fluid. The drilling fluid had good anti-fouling performance，inhibition performance，lubrication performance and reservoir protection performance，which could meet the requirements of field drilling.