In order to solve the problem of wellbore instability in drilling process，the hyper-branched polyethyleneimine（HPEI）with weight average molecular weight of 6×10⁴ g/mol was selected as a shale inhibitor for water-based drilling fluid. The inhibitive performance on shale of HPEI was evaluated via bentonite inhibition texts，linear swelling experiment and shale hot aging recovery experiment. The inhibition mechanism of HPEI was analyzed by means of infrared spectroscopy，X-ray diffraction and thermogravimetric analysis. The results showed that HPEI exhibited excellent inhibitive property on the hydration expansion of shale. The yield point of suspension with 20% bentonite in 1.5% HPEI solution were only 0.5 Pa. The swelling hight of bentonite in 3% HPEI solution reduced by 67% than that in distilled water. The lower the pH value of solution，the higher the protonation degree of HPEI and the better effect of inhibition on the dispersion of bentonite were. The mechanism of HPEI inhibiting clay hydration expansion was described as follows. The HPEI molecules entered the clay layers and weakened the clay hydration through the common role of electrostatic attraction and hydrogen bonding. The hydrophobic structure of HPEI prevented water molecules from entering the clay layers and suppressed the expansion of bentonite crystal layer.