570 100。 According to the information in China Postal Code Library, Haikou City is located in Hainan Province, and its postal code is 570 100. China postal code adopts four-level six-digit coding structure, with the first two digits 57 representing Hainan Province. The third digit 0 represents Haikou City; The fourth digit 1 stands for Haikou City; The last two digits, 00, indicate the delivery office.