Cheng Yi was the representative of the Reason School on the study of I Ching during the Northern Sung Dynasty. The characteristics of his I Ching study was the application of reason to interpret the I Ching of the Neo Confucian School. According to Cheng Yi, ”I” was the book to convey truth, which the truth of the world has been included in I Ching and ”I Ching” has been communicable with “heavenly principles”. He interpret I Ching with heavenly principles and criticized the Numerology of I Ching and the study of the Reason of Wang Pi during the Han Dynasty. This paper has aimed at Cheng Yi's thinking of the study of I Ching, which demonstrated the Reason with astrology and explained I Ching with reason, or even introduced history into I Ching to state reason with history to do detailed analysis and investigation on treatment and Yin (negative) Yang (positive) etc., items to explain his own thinking and outlines on I Ching.