Based on the numerical simulation results and the geological records of regional environmental evolution and differentiation in China, the changes of the Asian monsoon system at ca 2.6 Ma BP has been analyzed. The analysis shows that, at ca 2.6 Ma BP, the Asian monsoon (including winter monsoon and summer monsoon) was remarkably intensified. The prevailing wind direction of winter (northwest) monsoon in northern China shifted from west-north-westerly to north-westerly and its influencing area extended gradually from Northwest, North and Northeast China to Central, East and South China. However, the influence of southwest monsoon on Northwest China had been weakened. Before 2.6 Ma BP (e.g. in Pliocene), the Tibetan Plateau Region was dominated by subtropical evergreen forests and the Northwest China was not as dry as that today, suggesting that the southwest monsoon could penetrate on and cross over the Tibetan Plateau, and influenced the Northwest China. However, after 2.6 Ma BP, the Northwest China became drier than that before, reflecting the weakening of southwest monsoon influence on the Northwest China. This is the result of the uplift of the Tibetan Plateau to an important critical height (perhaps 2000m), which gave rise to the changes of the Asian monsoon system.