Abstract: The majority of the hidden coastal sedimentary strata in Jiangdong New Area, Haikou City, from the Haikou Formation (N₂h), are mainly composed of semi loose and semi diagenetic shell clastic rocks and shell glutenite. The pores are extremely developed, with strong permeability, and it is easy to produce underground cavities, leading to engineering geological problems such as cavity collapse. Based on this, the spatial distribution characteristics of the Haikou Formation were firstly analyzed, and then, the basic physical and hydraulic properties of two types of rocks as the research objects were tested. The mechanical tests were conducted separately under natural and saturated conditions to study the engineering geological characteristics of the rocks. The preliminary analysis on the distribution characteristics and causes of the underground cavities were also conducted. The results show that: (1) The burial depth range of the roof of the Haikou Formation is 4.6-44.4 m, gradually being shallower from northwest to southeast, and the central region is shallower than the north, west, and south regions. The two types of rocks are mainly distributed in the first and third sections of the Haikou Formation; (2) There are many pores and cracks inside the two types of rocks, with average uniaxial compressive strengths of 24.82 and 21.46 MPa, and average softening coefficients of 0.58 and 0.51, respectively. The softening effect is very obvious when encountering water; (3) The characteristic of easy dissolution of two types of rocks leads to the formation of dissolution cavities in their distribution areas, mainly developed in the starting area of Jiangdong New Area and the Yunlong-Yanfeng area. Cavities form during the process of sedimentary cementation of two types of rocks and then the dissolution of groundwater leads to further development and expansion of the cavities. This study can provide reference for the engineering construction of Jiangdong New Area in Haikou city.