The prominent capacities of 5G New Radio (5G NR) cellular networking drive the rapid development of many fields. The ubiquitous implementations of 5G NR cellular networking also provide swarm Unmanned Aircraft System (UAS) networking the feasibility of scalable deployment and smart control. However, the conveniences derived from 5G NR also bring other vulnerabilities to swarm UAS networking. The advanced capacities of 5G NR enable attackers to commit disruptive attacks to swarm UAS networking more severely and quickly. The requirement for the security of swarm UAS networking is imminent.In this paper, we propose a lightweight Blockchain-based secure routing algorithm for swarm UAS networking. We leverage the lightweight Blockchain to enhance the security of routing of swarm UAS networking which is based on 5G NR cellular networking. Different from the conventional routing algorithms, the proposed algorithm with lightweight Blockchain can avoid the malicious connections from attackers, recognize the malicious UASs and mitigate the attacks from malicious UASs. Concurrently, the proposed algorithm are swarm UAS oriented which aims to extend the deployment of swarm UAS networking on a large scale. Compared with Proof-of-Work (PoW) and Proof-of-Stake(PoS), we adopt pheromone to estimate the traffic status of each UAS in swarm UAS networking, construct consensus for swarm UAS networking with Proof-of-Traffic (PoT) and synchronize the updated blocks for lightweight Blockchain passively under the constraints of energy consumption. The evaluation shows PoT can reduce the routing consumption in the processes of consensus construction and blocks synchronization. Compatible with the constrained resource of swarm UASs, the lightweight Blockchain-assisted approach, proposed in this paper, can maintain the efficiency of swarm UAS networking.