Mega projects require large funding,which often reaches tens or even hundreds of billions of yuan.In the state-oriented financing system,the huge fund gap has generally become an important factor restricting the development of mega projects worldwide.Since the 1980s,China's long-span bridge projects have developed rapidly as typical mega projects as their financing systems have also undergone a series of changes and shown a high adaptability to complexity.However,the existing literature based on the perspective of evolution lacks focus on the complexity of mega project financing systems and its causes.Therefore,the complexity of the financing methods employed in the financing systems for China's long-span bridges is analyzed in this work from the dimensions of time and regions through investigation and interviews.On the basis of the results and complex adaptive systems theory,the causes of the complexity are further explored in terms of the clustering effects of financing subjects and the deep uncertainty of financing environments.Then,the evolution rules of financing systems of mega projects are established from the aspect of financing subjects,financing environments,and financing methods.The research results can enrich decision-makers'understanding of the essential attributes and formation mechanism of the financing systems of mega projects and provide powerful support for decision-making in financing.