This study dynamically deconstructs the implicit debt risk of urban rail transit PPP projects based on grounded theory, revealing the nonlinear transmission path of "institutional constraints → contract alienation → credit nesting → fiscal pressure", breaking through the traditional static analysis paradigm. Through multi-source data fusion (policy texts, in-depth interviews, project contracts) and three-level coding analysis, it was found that implicit debt risk is a composite product of institutional trust loss, risk return imbalance, and credit leverage alienation. Its core mechanism is reflected in the synergistic effects of rigid fiscal payments, asymmetric risk allocation, and nested transmission of financing structures. Research and construct a dynamic governance framework under the triple constraints of "system contract credit", proposing that the institutional level needs to improve the demonstration of dynamic fiscal capacity and coordinate with support policies for small and medium-sized enterprises. Contract design should implement flexible clauses of "excess return sharing gap risk sharing", and financing transmission should isolate government credit and innovate asset securitization tools. This framework provides a theoretical paradigm for resolving the conflict between the goals of "stable investment" and "risk prevention", and helps transform the PPP model from debt driven to value co creation. It has decision-making reference value for the high-quality development of infrastructure during the 14th Five Year Plan period.