Through investigating the initiation and the vicissitudes of medieval monasticism and mysticism, this essay analyzes the penetrating relation between monasticism and mysticism: mysticism is the spiritual core of monasticism, and monasticism is the institutionalized foundation of mysticism. Due to the co-operation of monasticism and mysticism, "monastic-mysticism" was constituted in the Middle Ages. Monasticism and mysticism, or "monastic-mysticism", always lives in a dynamic tension with the hierarchy and legislation of the church. The relation between medieval monasticism and mysticism has great significance for understanding and resolving the complicated problems of modernity.