With the rapid growth of unmanned aerial vehicle (UAV), there is now an increasing interest in the security of UAV systems. There exist various attacks which have threatened the security of UAV systems, in terms of the global navigation satellite system (GNSS) spoofing. Consisted of multiple UAVs, the flying ad hoc networks (FANET) have been studied to extend the employment and coverage of UAV systems. Based on the connectivity and information interaction of FANET, in this article, the blockchain technology is applied to detect GNSS signal attacks for UAV systems. In particular, we first introduce an overview of the GNSS security of UAV systems and the principles for blockchain technology. Based on the principles, a logical architecture is proposed, where blockchain is taken into consideration for GNSS spoofing detection. Performance analysis verifies that the proposed GNSS spoofing detection system can be used effectively. Finally, several challenges in blockchain are discussed, including security, cost, and regulation.