Ou-yi's Explanation of the Four Books (Si Shu Ou Yi Jie) is a work that annotates the Four Books which are canonical books of Confucianism. Master Ou-yi Zhi-xu (1598-1655 A.D.) wrote this book to Bhikkhu Che-yin, one of his students, to highlight the Buddhist Paramārtha-satya (absolute truth) through the explanation of scripts of the Four Books. A Straightforward Explanation of Doctrine of the Mean (Zhong Yong Zhi Zhi) is part of Ou-yi's Explanation of the Four Books, which aims to make known the Buddhist learning of mental nature. Through the analysis of A Straightforward Explanation of Doctrine of the Mean, we learn that Master Zhi-xu explained scripts of the Doctrine of the Mean (Zhong Yong) with the teaching classification of T'ien T'ai Buddhism and the thought of ”Return to the Gate of Bhutatathata (True Suchness) via the Gate of Arising and Ceasing” in Mahāyānaśraddhotpāda-śāstra (Awakening of Faith in the Mahāyāna, Da Sheng Qi Xin Lun). Furthermore, this book also realizes the communication between T'ien T'ai Buddhism and Mahāyāna-śraddhotpāda-śāstra as well as the communication between the argumentations of Confucianism and Buddhism through the study order of attaining perfect realization, carrying out perfect practice and achieving perfect status. This essay mainly carries out exploration and analysis on A Straightforward Explanation of Doctrine of the Mean and discusses the approaches that Master Zhi-xu adopted to interpret the Doctrine of the Mean and their characteristics.