To probe the rule of nitriles hydrolysis in high temperature liquid water with carbon dioxide, the effect of carbon dioxide pressure on phenylacetonitrile hydrolysis in high temperature liquid water was investigated, and the effects of carbon dioxide pressure and reaction temperature on pH of reaction system were calculated. The results show that the rate constants of hydrolysis at 0, 0.2 and 0.4 MPa carbon dioxide pressure were 6.2 × 10-4，4.1 × 10-4，3.0 × 10-4 min-1 at 473.15 K, respectively. And the corresponding pH values were 5.6、4.1、3.9. Since the base-catalytic mechanism is dominant in phenylacetonitrile hydrolysis, carbon dioxide can’t promote the hydrolysis of phenylacetonitrile efficiently.