Abstract: Third-party market cooperation conforms to the trend of globalization and meets the common demands of the development of all countries. However, the unprecedented changes in the past century have had a great impact on the world economy. The threshold of enterprise cross-border project financing risk premium has been raised, external financing costs have increased, and the lack of common information between entities has become increasingly serious, which further aggravates the risk aversion perception of cross-border investment by various enterprise entities. Third-party market cooperation entities have low willingness to invest and finance, and the problem of weak participation is gradually highlighted. The financing problem determines the form of division and combination of cooperation participants, and is the key issue to explore the third-party market cooperation. Therefore, taking Chinese enterprises, enterprises in developed countries and enterprises in the third market countries as the main body, a tripartite evolutionary game model is constructed based on the portfolio theory of investment factors, and four third-party market financing cooperation modes divided by the strategic alliance theory correspond to eight strategies of the game model, describing the third-party market cooperation body and financing cooperation mode. By exploring the key factors that affect the choice of the third-party market cooperation mode, analyzing the interest relationship between the subjects, and then discussing the financing game behavior and strategy choice of the third-party game subjects in different stages of cooperation. Finally, a numerical example is simulated. The research finds that: in the initial stage of cooperation, the financing cooperation mode is chosen as two parties undertake construction and one party contributes, the participation of Chinese enterprises and enterprises in developed countries is low, and the capacity incentive, the advantages and disadvantages of the host state-owned business environment and the risk allocation mechanism are the key factors affecting the game results in this stage; In the stable cooperation stage, the financing mode is selected as one party invests, one party undertakes construction, and one party assists. The incentive of knowledge and technology spillover from developed country enterprises to Chinese enterprises and the negotiation cost of active participation of developed country enterprises under the low strength guidance strategy of Chinese enterprises are the key factors affecting the game results in this stage; In the stage of strategic cooperation, the financing cooperation mode has reached the ideal state of joint investment and joint construction, but it is still negatively affected by the negotiation cost and information asymmetry risk under the active strategy, especially the enterprises in the third-party market countries are more sensitive to cost and information. The research conclusions provide model reference and stable strategies for third-party market cooperation at different stages, and also provide countermeasures and suggestions for building high-quality the Belt and Road and improving the third-party market financing cooperation mechanism.