Abstract: The 1605 Qiogzhou great earthquake is considered to be the only great one that occurred in the Hainan Island. It might have particularly far-reaching implication to further study this great earthquake and would be good for the analysis of the seismic risk in this area as well as to give a better elucidation of the seismic characteristics and structures in the northern margion tract of the South China Sea.Field investigations were carried out in detail by the authors. A lot of genealogies,inscribed steles and local county records which suggest the condition of the earthquake and the damage wrought and the large-scale land subsiding into the sea caused by the earthquake were found. This study enables us to identify the intensity in the epicentral area which could reach up to XI degrees,the magnitude could be 7.5-8 and the location of epicenter of the 1605 Qiong-zhou great earthquake might be near Tashi of Qiongshan county (20N,110. 5E).Study of the geotectonics and the geophysical field in the focal area indicates the seismic faults could be the Guangcun-Puqian Fault striking ENE and the Tashi-Yanfeng Fault striking NNW (the former served as the main axis). This earthquake is regarded as the result of the joint action of both the horizontal and vertical structural stresses (the main compressive stress axis WNW 300); but the vertical atress played the main role in the process of the earthquake and the subidence of the land.It has also been shown by the processing and calculation results of gravity and magnetic anomaly data that the 1605 Qiongzhou great earthquake is considered to be the composite result of the difference in material density at different structural blocks,the difference of gravity fields,the different movements in the upper mantle,magma materials and thermodynamics,caused by the gravity isostatic compensation and adjustment processes in certain tectonic setting of the deep geological field,not the result of the crustal fault activeties only.