Bordering Dongtai to the north, Rudong to the south-east, Rugao to the south, Taixing to the south-west, and Jiangyan to the west, the city is located on the south-western shores of the Yellow Sea.

They are Hǎi'ān (海安), Lǎobàgǎng (老坝港), Jiǎoxié (角斜), Lǐbǎo (李堡), Xīchǎng (西场), Dàgōng (大公镇), Chéngdōng (城东), Sūnzhuāng (孙庄), Yǎzhōu (雅周), Qūtáng (曲塘), Hújí (胡集), Nánmò (南莫), Báidiàn (白甸) and Dūntóu (墩头).

In 411, Haining (海宁, not the one in Zhejiang), a county administered the area of Hai'an nowadays and its environs was founded.

Fourteen years later, it was reduced to town status and annexed by Hailing county, then Taizhou.

During the second Sino-Japanese war, Zishi (紫石) county was founded by the communist government there in 1943, it was renamed Hai'an in 1948.