For the underground construction of coastal cities in China, there is an urgent need to accurately position unfavorable geobodies such as faults and boulders. Based on the geological characteristics of coastal cities, this study conducted 3D numerical simulations using a high-density resistivity method, determining the effects of the electrical properties and thickness of the overburden on the survey results, as well as the DC electric field characteristics varying with the sizes and burial depths of detection targets. The results show that the resistivity difference between the overburden and the targets serves as a critical factor in determining the influence of the overburden. For low-resistivity fracture zones, a higher resistivity of the overburden signifies more prominent responses from the fracture zone. Under middle- to high-resistivity overburden conditions, shallowly buried boulders can be easily found, and larger boulders exhibit more significant high-resistivity characteristics. In the exploration along the Qingdao metro line 5, the high-density resistivity method played a vital role in exploring fracture zones and boulders, verifying the effective application effects of the method. The results of this study provide a basis for selecting engineering exploration methods and determining operating parameters in coastal cities.