Abstract: Potassium silicate/polyvinyl alcohol inhibitor was selected to protect the borehole instability in foam drilling, and its inhibitive properties were evaluated by means of shale self-priming experiment, mud-making inhibition of bentonite, rolling recovery experiment and hardness test. The results indicate that potassium silicate/polyvinyl alcohol has a good inhibitive properties. Finally, the inhibition mechanism of the inhibitor was studied by XRD. The results showed that the strong inhibition of potassium silicate/polyvinyl alcohol to the shale is derived from the synergies of potassium silicate and polyvinyl alcohol, in which sealing property and film- forming ability are the major factors to maintain borehole stability.