近五百年前，明太祖朱元璋颁布了一项海洋政策，从此，中国的万里海疆变得一片沉寂。五百年后，西方侵略者用大炮轰开了清王朝仍旧紧锁的国门。

朱元璋颁布的这项政策内容是什么？是什么样的原因使他实行这样的海洋政策的？这项政策为什么能在中国实行五百年之久？纵观我国海洋这近五百年兴衰沉浮的历史，我们还得从海禁这项政策细说。

Nearly 500 years ago, Zhu Yuanzhang promulgated a ban on maritime trade. From then on, few foreign maritime trades were seen on China's large seas. 500 years later, western invaders forced Qing Dynasty to open China by means of cannons.

What was the content of the ban promulgated by Zhu Yuanzhang? Why did he enact such a ban? Why can this ban be implemented in a period as long as 500 years? In order to make a comprehensive view of the ups and downs in this 500-year-old history, we have to study this ban on maritime trade.