In this paper, the authors designed and optimized a starch graft polymers mud system used for drilling in water-sensitive strata, using the graft copolymer of starch as filtrate reducer, selecting inhibitor, anti-sloughing agent, lubricant, encapsulating agent and bentonite as treating agents. Tests show that the system’s API filtrate loss is 7 ml. The reduction of relative expansion is 84.2%. Recovery of cuttings is 92%. And the friction coefficient is 0.07. This mud system has achieved good results in real drilling in water-sensitive strata.