The well-known paradox between Marxism and morality is that on the one hand, Marx claims that morality is a form of ideology that should be abandoned, while on the other hand, Marx makes quite a few moral judgments in his writings. It is in the research after Marx's death that the paradox is found, explored and solved. This paper surveys the history of interpreting Marx from the aspect of moral philosophy by dividing it into three sequential phases. Then it presents the research on Marx in each phase, points out conflicting questions within the different periods and puts forward the solution in the end. This paper points out that a philosophical viewpoint based on Marx's theory of historical materialism is the key to solving the paradox between Marxism and morality.