It is a consensus in bioethics that human dignity can never be violated by clinical decision-making as well as medical research. However, when people have access to the principle of human dignity in specific medical practices, they could draw very different conclusions. By tracing the history of the concept of dignity, we can find that the basic connotation of dignity is superiority. According to the rationalist tradition, human beings are elevated above other creatures and thus have dignity because of the autonomy possessed. However, the autonomy-dignity model is based on the "strong" side of humanity, which would inevitably take the vulnerability of human nature lightly and result in discrimination of the vulnerable. In fact, vulnerability is also an aspect of human nature. After World War II, scholars' emphasis on the universal human dignity is a response to vulnerability. In bioethics, respect for the vulnerable has gradually become an important principle. Christian thought about finitude and dependence of human nature inspires us to think about vulnerability in the metaphysical sense. The Christian analysis of spiritual freedom and the finitude of human nature is an important resource for us to understand the concept of dignity from the perspective of a comprehensive human nature. According to Reinhold Niebuhr, it is the refusal of finitude and vulnerability of human nature that makes human beings fall into sin. In this sense, the principle of respecting the vulnerable and the warning of "Don't Play God's Role" contain great implication. Autonomy and vulnerability are two aspects of human nature, based on which dignity not only can protect the vulnerable, but also can show the superiority of human reason.