Abstract: The use of nuclear power sources in outer space is the inevitable choice to meet the needs for the exploration of deep space. Because of its inherent risk factors and the widespread influence of space activities on the Earth, how to use the nuclear power sources safely is a problem faced by the whole international community.At present, only two international instruments directly regulate the use of Nuclear Power Source in outer Space, namely the "Principles Relevant to the Use of Nuclear Power Sources in Outer Space" adopted in 1992, and the "Safety Framework for Nuclear Power Sources Applications in Outer Space" established in 2009. These two instruments provide general principles and specific technical specifications as well as appropriate safety standards to be followed. However, both of them are considered as soft law, which are not legally binding and enforceable. The possibility of forming a new space law treaty on Nuclear Power Sources seems very low for the moment, the recourse to some kind of soft law, such as Principles, Framework, or Code of conduct, is an interim positive and realistic solution. It can be a good way to harmonize national laws and develop an international regime on the specific issue of NPS.