Zhang Zai's "Great Harmony" in his "Correcting Youthful Ignorance" is a significant text to understand his theory of the Dao of Heaven. However, there have been different interpretations of the relation between the important concepts of "Great Void" and "Vital Force" in the text, which is crucial in understanding Zhang's theory. In this article, we re-examine four major interpretations of the relation between the two concepts, considering them as reasonable but not adequate in deciphering the original meaning of Zhang. The author proposes a synthetic interpretation, arguing that Zhang's original meaning embodies a synthesis of the meaning of "Great Void" and "Vital Force" being neither the same nor different and that of "Great Void" being a form of "Vital Force".