Oleate graft copolyammonium (OGCA) was prepared and its performance as a shale inhibitor was evaluated by bentonite linear expansion test, anti-swelling experiments, mud ball experiments in water-based drilling fluid. Anti-swelling results shows that anti-swelling rate of OGCA reaches up to 89.7% and OGCA has strong inhibitive capability to bentonite hydration swelling confirmed by the result of mud ball experiment. From the results it was found that both OGCA and modified starch have an excellent synergetic defiltration effect on the water-based drilling fluid. The good performance of OGCA as a shale inhibitor should be contributed to its good capacity of controlling the particle size of bentonite. The inhibition mechanism of the polyamine salt was analyzed by thermogravimetric analysis and scanning electron microscope.