By introducing the definition of propensity categories of relations, the implicit information in the knowledge graph is mined and the MCBE (Maximum Clique Based Expansion) algorithm is used for data expansion. Experimental results show that with baseline models TransE, RotatE, HAKE and Complex on FB15K dataset, its MRR and Hits@1 metrics are improved by (7.9%, 9.6%), (4.2%, 3.3%), (2.7%, 4.8%) and (1.7%, 2.4%), respectively. Experiments are also conducted on the FB15K, YAGO3-10, NELL-995 and DBpedia50 datasets using the TransE model as a baseline, and its MRR and Hits@1 metrics are improved on the above datasets by (7.9%, 9.6%), (0.3%, 27.7%), (20.1%, 100%), (4%, 38.7%), respectively. Finally, the MCBE algorithm is applied to the self-constructed knowledge graph of “anti-drug law” and its MRR and Hits@1 metrics are improved by (10.6%, 12.3%). The experimental results show that MCBE algorithm improves the prediction accuracy of legal knowledge graph.