Haikou lies in in the north of Hainan Island. It borders on Qiongzhou Strait in the north, and is separated from the mainland for about 18 nautical miles. It is contiguous to Qiongshan (a city) in the southeast, and is close to Chengmai County in the west. The longest part is 25.6km from east to west, and the widest part is 15.1km from north to south. The circumference of the border is 84.75km, and the total area is 244.55sq km. The planned area is 1127sq km, and construction area of modern city is 34sq km. Haikou is plain of the seashore for the most, and mainly consists of new and old deposits. High in the south and low in the north, it is low and flat.