Abstract: The soft law in outer space is the non-legally-binding outer space law that contains actual executive effect. It cannot be categorized as treaty or custom, and it is usually resolution or declaration of international organizations and conferences. With the development of various space activities, the evolution of soft law in outer space can be divided into four main phrases with their own historical characteristics. Completing the legal regime of outer space law, the outer space soft law has boosted the development of treaty and custom in outer space law, and it is also the complementary regulation when no treaty or custom can be applied.