•Block-DEF is proposed to solve big data and privacy challenges with a loose coupling design.•The blockchain bloat is avoided by combining a mixed blockchain structure with an O-NPBFT mechanism.•The traceability and privacy of evidence are balanced by using multi-signature schemes. A secure digital evidence system should ensure that evidence cannot be tampered with and that private information cannot be leaked. Blockchain, a distributed tamper-resistant and privacy-preserving ledger, provides a promising solution for decentralized secure digital evidence systems. However, due to the huge number of digital evidences and the contradiction between the traceability and the privacy of evidence, blockchain faces big data and privacy challenges. To solve the above issues, we propose a secure digital evidence framework using blockchain (Block-DEF) with a loose coupling structure in which the evidence and the evidence information are maintained separately. Only the evidence information is stored in the blockchain, and the evidence is stored on a trusted storage platform. To avoid blockchain bloat, a lightweight blockchain combining a mixed block structure with an optimized name-based practical byzantine fault tolerance consensus mechanism is proposed. To support the traceability and the privacy of evidence, the multi-signature technique is adopted for evidence submission and retrieval. The analytical and experimental results show that Block-DEF is a scalable framework, it guarantees the integrity and validity of evidence, and balances privacy and traceability well.