In this article, the inhibitive properties of polyether diamine and formate salts (mainly sodium formate and potassium formate) were compared through a bentonite inhibition test, a shale hot-rolling dispersion test, and a particle distribution test. The inhibitive mechanism of polyether diamine and formate salts was analyzed via zeta potential measurement, X-ray diffraction analysis, modified montmorillonite water adsorption test, and activity test. The results indicated that, compared with formates, polyether diamine can reduce the interlayer spacing of hydrated clay utmost and lead to clay dehydration with low concentration. Meanwhile, adsorption of polyether diamine onto a clay surface renders the clay surface more hydrophobic, while formate salts prevent clay hydration mainly through lowering the solution activity.