After more than three years’ long drawn fight，the Brexit Agreement has officially been approved，and UK will leave EU by January 31，2020. In the short term，Brexit has a negative impact on Portugal in many aspects，such as tourism and investment. However，in the long run，Brexit has provided new opportunities for Britain and Portugal to maintain and consolidate their historical ally relations，and Portugal’s strategic position will be highlighted，which will help Portugal to play an active role in the relationship among Britain，EU and other countries. This paper analyzes the influence of Brexit on Portugal from three aspects：the process of Brexit and the position of Portuguese government；the influence on Portugal at different stages of Brexit；the current and future relationship between Britain and Portugal.