Xi Jinping’s ecological civilization contains the ecological wisdom in Chinese traditional philosophy. Mencius’thought of the relationship between man and nature undoubtedly has important research value. Mencius said that Tian includes the Tian of God, the Tian of fate, the Tian of Yi Li (morality) and the Tian of nature. Mencius’attitude to different Tian is different. First, Mencius considered that we should enjoy the Tian of God, revere the Tian of God and pay attention to Ren (people). Second, Mencius considered that we should obey the Tian of fate, know the Tian of fate to fight against it. Third, Mencius considered that we should devote our hearts and souls to the Tian of Yi Li, and undertake the mission of it. Finally, Mencius considered that we should obey and know the Tian of nature so we can use it. Therefore, Mencius not only has the thought of “harmony between man and nature”, but also has the thought of“Tian and Ren are two different parts”. To accurately grasp Mencius thought of the relationship between Tian and Ren, we must follow the principles of clear allocation and dynamic condition. A healthy ecology leads to a prosperous civilization while an unhealthy one will ruin a civilization. Today, the“harmony between man and nature”ecological gene is still the cultural core of ecological civilization construction in China.