This article analyzes the main content of Sun Yat-sen's theory that knowing is more difficult than doing from the areas, the order, the knowledge and the relation of knowing and doing. Based on this, the paper further discusses the main contribution and limitation of the theory. The author thinks that the theory not only has the value of innovation in the Chinese philosophical history and but also has the basic orientation of materialism. At the same time, the theory has metaphysic nature and absolutized tendency for the restriction of that time.