This dissertation aims at to sum up and analyze Karl Marx and Fredrick Engels' critical theory of capitalist political system. To response the challenge that Marxism political theory is facing and the need of the theoretically guidance from socialist practice, make the research of Marx and Engels' critical theory of capitalism emergent and important. On the basis of criticizing the two viewpoints in the present research of political theory of Marx and Engels', that is, their political theory does not exist at all or their political theory is no more than the theory of proletarian dictatorship, it demonstrates that the function of their capitalist political system theory in the course of the forming of their whole theory and its position in their whole theory. It begins with the fundamental part of Marxism political theory, also the theoretical base of this research, that is, theory of classes and state, then sums up their theory of capitalist political system as follows: first, the forming of capitalist political system, mainly containing the process that capitalism and capitalist political system formed, in which the major classes gamed, focusing their two views being neglected in present research, i.e. the roles the classes of workers and peasants played, and the political conditions the capitalism needed. Second, the capitalist parliament system, mainly introducing their thoughts of the necessity, rationality and its class nature, especially the new thoughts in their late years. Third, the capitalist election system, analyzing their ideas of the progressiveness and limitation of the election system, stressing their exposition about the decisive factors influencing election. Fourth, capitalist party system, in this part, it discusses the historical background, under which their theory of party system formed, main viewpoints including the nature, function and changes of the parties, also the existing situation that the proletarian party under bourgeois party dominating system. From the angle of theory innovation, the dissertation points out its two major methodologies, historical materialism and dialectics and the practical feature of the methodologies, also it takes analyzing the critical, historical traits of Marx and Engel's capitalist political theory as one of its main tasks. In the end, it discusses the time significance of Marx and Engel's capitalist political theory from both the angles of socialist and capitalist historical practice.