Li Erqu is one of the three greatest Confucian at the beginning of the Qing Dynasty. His epistemology and cultivation theory inherit and develop Zhang Zai who put people’s inside and outside together, and put forward one’s heart makes appropriate and fast response to the changing environment outside. Therefore, he thinks the real Confucian has to build his spirit inside and struggle to become a ruler in the real life at the same time. Based on the old version of Great Learning, Li Erqu thinks the core of this book is “Mingtishiyong”, the first step of learning is “Gewu”, and the guiding principle is “Huiguozixin”. Li Erqu’s mode of interpretation reflects he combines Neo - Confucianism with the Lu - Wang Xin Xue. To an extent, Li Erqu’s epistemology of the Great Learning tallies with some information construction theory in contemporary information philosophy.