The conception of the Parent King did not originate from Confucius, but from a popular viewpoint both in the private sector and the ruling class before the Spring and Autumn Period. Because of the dominance of Confucianism, this conception has lasted for thousands of years, forming an implicit foundation for li (禮) of an individual and the State. In light of the newly published text of Chu Slips of the Parent King by Shanghai Museum, this paper examines its detailed interpretation by Confucius, which was important enough to be buried in tombs in the pre-Qin era, together with a number of other recently discovered pre-Qin slips, as well as other existing materials of popular pre-Qin Confucian political thoughts, so as to explore whether the concept is in parallel or conflict with western constitutionalism, which sets forth limited governments. The Parent King concept, exemplified in the analysis of "five reaches," "three withouts," and "five effects," is completely compatible with the central theme of Confucianism, imposing the king's moral duties toward the subjects, and meanwhile confers upon the king a moral identity toward the subjects. It, however, fails to explain why a king would be willing to undertake his moral duties, and says nothing about whether the king can retain his moral identity if he fails to fulfill his duties. The Parent King concept basically sets forth for kings a benchmark of good deeds towards the subjects to cope with the political reality in which kings are guilty of wrongdoing. The methodology does not deviate from that of optimistic moral idealism. The Parent King concept paves the way to the paternalism government, closes the concept gap between the heavenly chosen king and the biological father, and champions the advocates for kinship politics in their long-term debate with those who espouse having an elite to reign as the successor to a deceased king instead of the king's son. Both the Parent-King concept and western constitutionalism are to find solutions to eliminate evils of the monarch system, but with different answers for the solutions. The Parent-King concept is a compromise offered by the advocates in light of the very existence of monarch kings, and the compromise underscores the major difference between the concept of people being the foundation of the State (民本思想) and that of democracy. The thinker behind the Parent-King concept decided to "dance" with the monarch, recognized its legitimacy and prescribed an "ought-to" list for the monarch, while democracy seeks to abolish the monarch system. The concept may lead to perpetuation of inequality between the monarch and the subjects, thus distinguishing it from what western constitutionalism considers fundamental-man being created equal. Another major difference lies at the ultimate significance of popular support in the legitimacy of government power as emphasized by western constitutionalism. The Parent-King concept pulls away the theoretical pillar of people's will as the determining factor in identifying the heavenly choice of human kings, thus hindering the possibility of conceptual development in establishing a government by the people. Moreover, a Parent King, stepping into the shoes of a parent, will not step down from the post voluntarily, nor would have periodical elections associated thereto. The concept does not create any room for development of a regime of periodical elections to curb the abuse of power. In sum, the passage of Parent King may not have occupied a prominent place in the Confucianist materials in discussions of li (禮); it, however, plays a pivotal role for us to understand why modern constitutionalism was not fully developed in ancient China.