Based on previous researches, combining the background of the times, Karl Marx and Friedrich Engels demonstrate the essence, characteristics, the main carrier and social function of the public opinion from the perspective of social development. As a form of communication, the formation of public opinion has spontaneity and universality. But at the same time, public opinion can represent and reflect the interests of a certain class, class and group, and its demands. It has certain instability, and can be influenced by external factors, such as war, interests, and the words and deeds of rulers and social activists. Moreover, the expression of public opinion is associated with political consciousness development. The press is the main carrier of public opinion. Marx and Engels made good use of newspapers and magazines to influence and guide public opinion. In their view, the premise is that the newspapers expresses and reflects public opinion in an authentic, objective, and accurate way. In terms of the social function of public opinion, Marx and Engels think public opinion as a universal, covert, and coercive force.