Aimed at the problems of non-ideal rheological property，high filtration，insufficient stability，poor compatibility with conventional additives，great damage of filtrate to reservoir permeability in the application of environment-friendly water-based drilling fluids，a natural biodegradable polymer-modified tamarind seed polysaccharide（M-TSP）was prepared using maleic anhydride，tamarind seed polysaccharide（TSP）and triethylamine as main raw materials. M-TSP was added to the bentonite suspension with mass fraction of 4.5% to obtain polymer bentonite suspension，the rheological property，stability，filtration，compatibility，and damage to the core of polymer bentonite suspension were carried out. The results showed that the bentonite suspension added with 1% M-TSP had good rheological property and stability. bentonite suspension added with 1% M-TSP had good compatibility with the conventional additives of environment-friendly drilling fluids. when adding 1% M-TSP，the initial filtration time of the polymer bentonite suspension was 121 s and the filtration was only 1.7 mL within 30 min，showing good filtration loss performance. The damage rate of M-TSP to sandstone core was only 2.06%— 3.23%，indicating that M-TSP had less damage to core. The M-STP is promising to be applied in environment-friendly water-based drilling engineering.