In this paper, we focus mainly on the ideology of democratic socialism, the principle of the Socialist International. Through historical approach, the ideology’s main thought and its development were examined. It was found that democratic socialism came from Marxism and went another way in the Second International. The Fabian Society, Karl Kautsky, Edward Bernstein all contributed to adjust socialism to a democratic way. Although there were differences between their ideas and thoughts, they all insisted the same goal that socialism must be practiced by democratic way.