Abstract: In the context of China's new power system construction, the traditional mid- and long-term transaction mode with fixed guaranteed-output requirements cannot accommodate transactions on renewable energy, which is characterized by significant variability and randomness. This incompatibility often results in reluctance in power delivery sales. To this end, this paper proposes a new power market mechanism for mid-and-long-term electricity transactions among provinces. It adheres to shared backup, shared peak load management, and shared risk at the receiving end. The aim is to facilitate large-scale renewable energy delivery and optimize resource allocation through additional measures. Based on the probability-output rate curve, an elastic transaction model considering both the potential undelivery of sellers and default of buyers is presented in this paper. Supportive models are established for determining floating-energy ceilings, market clearing, and deviation assessment. Simulation results demonstrate that this mechanism enables a stable export base through fixed electricity sales while enhancing market vitality through floating energy sales, leading to improved regional consumption of renewable energy in inter-provincial mid- and long-term electricity transactions.