Under the background of the construction of socialist rule of law with Chinese characteristics, with increasingly deepened study of the customary law, the theory of customary law has gradually improved, and judicial practice has also extended from the field of civil and commercial affairs to criminal cases. This article analyzes the customary law of minority ethnic groups from the perspectives of legality, harmlessness and rationality.By analyzing the relevant cases of complementary customary law and conflict customary law of minority ethnic groups, we seek operability and legitimacy for the customary law of minority ethnic groups in the judicial field through reasonable adoption, flexible application and resolute prohibition, so as to improve the judges'awareness of minority law in their judicial practices. The awareness of national law provides a solid guarantee for judicial practice.