Abstract: Residual soil from decomposed basalt at Leizhou Peninsula is special regional soil. It has poor physical properties of high liquid limit and large void ratio, good mechanical properties of high strength, and extraordinary properties of strong contractility, humidification and disintegration. This paper attempts to examine the mechanism of the special engineering geological characteristics of the basalt residual soil. It studies the mineral composition, chemical composition, structure, and pore distribution from laboratory tests. Results show that distinct aluminum, iron accumulation, and salt leaching processes occur during the development process of the basalt residual soil. Clay minerals are mainly composed of kaolinite, gibbsite and illite. Furthermore, they are rich in free iron oxide and free aluminum oxide. Microstructure is mainly clot and flocculation structure. The free iron oxide acts as coating that clads particles to form aggregate, which enhances structural strength of soil. Pores mainly consist of denudation pores and micropores in the aggregate and flocculation, and with larger specific surface area and adsorption energy. Large surface tension occurs after immersion which leads to softening and disintegration. Therefore, close attention should be paid to engineering disasters caused by rainfall due to the water sensitive and poor water resistance of the basalt residual soil.