The concept of mind-heart (xin) is applied quite frequently in Mencius. This paper analyses the meaning of this concept by the means of classification. In the original context of Mencius, it mainly means psychological activities. However, a metaphysical tendency appeared in the interpretation of this concept after the Han Dynasty, which can be regarded as a result of the excessive interpretation of annotators according to their own philosophical positions. A correct understanding of the relation between mind-heart and things is essential to a reasonable interpretation of the concept of mind-heart.