In the research of the spectrum blockchain, more attention is often paid to how the blockchain architecture can achieve efficient spectrum access, neglecting the implementation details of "currency for spectrum trading". However, a robust security mechanism is crucial for the spectrum allocation system, and the potential economic risks brought by digital currency must be carefully considered. This article presents a comprehensive approach to address these concerns. Firstly, we propose a multi-entity spectrum trading system based on blockchain and mobile edge computing (MEC). Secondly, we introduce a stable spectrum currency called SCoin, which incorporates a "dual-token" function achieved through coupling the consensus mechanism with the spectrum market cycles. SCoin serves a dual purpose within the spectrum market as a stable transaction currency and an economic incentive. In addition, we introduce a set of stakes specifically designed for DPoS-based wireless blockchains. Considering the interference factor in stake assessment facilitates a direct evaluation of node effectiveness in the system. The simulation results show that our SCoin design can well maintain the stability of the SCoin’s price. In summary, SCoin provides a clear strategy that utilizes digital cryptocurrency to facilitate blockchain-based spectrum trading while enhancing economic security.