As part of the Brexit agreement，the Northern Ireland Protocol avoids the re-emergence of a hard border on the island of Ireland after Brexit by setting up an Irish Sea border. However，in future relationship negotiations focusing on economic and trade issues，the UK and the EU could not reach an agreement on the specific implementation plan of trade arrangements for Northern Ireland in the Protocol. The UK requested to renegotiate the Protocol while the EU took a strong stand and refused to compromise，resulting in negotiation deadlocks for several times. The Irish Sea border issue has aroused the dissatisfaction among the unionists in Northern Ireland，and violent riots broke out there as well. The Democratic Unionist Party demanded the abolition of the Protocol，and the political and social stability of Northern Ireland was severely challenged. The difficulties encountered in the smooth implementation of the Northern Ireland Protocol have become an important cause for the tensions between the UK and the EU and between the UK and Northern Ireland in the Post-Brexit era. Although changes in the political landscape of Northern Ireland after Brexit is conducive to holding a border poll，it is unlikely to realize the Irish reunification in the short term. The future economic development and social stability of Northern Ireland largely depend on the successful implementation of the Protocol between the UK and the EU.