Abstract: Kant’s ethics is based on the difference between two kinds of laws: natural law and moral law. However, in the process of Kant’s argumentation of moral laws or moral obligations, there is a complex situation: as far as the desire of human life itself (reflecting the internal purposiveness of natural life) is concerned, Kant believes that human reason should transcend it and pursue moral freedom other than aiming at one’s own happiness, or regard the performance of moral obligations as higher than the pursuit of one’s own happiness. On the other hand, Kant uses the natural law suitable for human’s natural life noumenon to demonstrate or prove the moral law. In Kant’s famous four cases of proving moral obligation, three cases are defended by natural law. Although these “natural laws” pay different attention to human organic life noumenon, they all start from such a natural noumenon. Therefore, what was rejected by Kant was called back by Kant. Kant’s ethical attitude towards the natural life noumenon and its law is a theoretical problem worthy of much attention.