Abstract: The factors influencing the pH of drilling fluid have been studied. The study results indicate that for fresh water systems, with the increase of the addition of NaCl, CaSO₄ and iron ore powder, the initial pH value decreased, but the pH value was not influenced by the addition of barite. Organic salt could increase the pH of fresh water and saturated salt water systems, and played as a buffer in saturated salt water systems. The additions of surfactant, HPAM, CMC and XC increased the pH value of both fresh water and saturated salt water systems. A strong buffering effect was observed for SMP-II in fresh water and saturated salt water systems, because phenolic hydroxyl and sodium phenolate react as a buffer pair.