Goshawk: a novel efficient, robust and flexible blockchain protocol. Proof of Work (PoW), a fundamental blockchain protocol, has been widely applied and thoroughly testified in various decentralized cryptocurrencies, due to its intriguing merits including trustworthy sustainability, robustness against Sybil attack, delicate incentive-compatibility, and openness to any participant. Meanwhile, PoW-powered blockchains still suffer from poor efficiency, potential selfish mining, to-be-optimized fairness and extreme inconvenience of protocol upgrading. Therefore, it is of great interest to design new PoW-driven blockchain protocol to address or relieve the above issues so as to make it more applicable and feasible. To do so, we present Goshawk, a novel hybrid consensus protocol, in which a two-layer chain structure with two-level PoW mining strategy and a ticket-voting mechanism are elaborately combined. We show that this newly-proposed protocol has the merits of high efficiency, strong robustness against “51%” attack of computation power, as well as good flexibility for future protocol updating. As far as we know, Goshawk is the first blockchain protocol with these three key properties. Last but not the least, this scheme has been implemented and deployed in the testnet of the public blockchain project Hcash(url{https://github.com/HcashOrg}) for months, and has demonstrated its stability and high efficiency with such real-world test.