Management of megaprojects is a typical complex system． The characteristics of megaproject complexity，and how to solve such complexity，are popular in the field of project management． A conceptual model is established to study the effects of complex factors，such as decision relevance，cognitive ambiguity and bias，and limited knowledge，on megaproject management． Then，in a complexity framework of decision relevance and incomplete information，the complexities associated with megaproject management decisions are divided into four types: “chaos”，“conflict”，“relevance”，and “hybrid”complexities． In accordance with those four types of complexity，this paper proposes a“complexity degradation”theory that consists of four basic logical strategies: complexity exploration，complexity absorption，complexity decomposition，and complexity tolerance． The complexity degradation theory reveals a complex self-organization law in the practice of megaproject management decision-making，and illustrates the endogenous and dynamic evolutionary characteristics of the complexities involved in megaproject management decisions． Finally，the complexity degradation theory is verified through the management decision-making practices of a deck pavement project for the Hong Kong-Zhuhai-Macao Bridge，which provides an enlightening illustration of the complexity degradation theory and its application to megaproject management．