flag of convenience
noun [ C usually singular ] UK uk/ˌflæɡ əv kənˈviː.ni.əns/ us/ˌflæɡ əv kənˈviː.ni.əns/
方便旗（指船只为逃避本国法规或赋税而悬挂的一种旗帜，表示其受他国税收或其他法律束缚）
If a ship sails under a flag of convenience, it means it is operated or taxed under the laws of a country different from its home country in order to save money.
They always register their ships under a flag of convenience. 他们总是在外国登记注册自己的船只，悬挂方便旗。