South China experienced multiple periods of extensional—compressional tectonic stress field reversals during the Late Mesozoic. The tectonic reversal from extension to compression between the Early Cretaceous to Late Cretaceous has been widely confirmed, but there is relatively little evidence of tectonic reversal during the Late Cretaceous. Hainan Island is located in the southern part of the South China Plate and is an overlapping area of the ancient Pacific tectonic domain and the ancient Tethys tectonic domain. It records complex tectonic—magmatic events and is a key area for studying the tectonic evolution of the continental margins of South China and the northern South China Sea. Methods:This study is based on the geothermal scientific exploration well FSR 1 deployed and implemented by SINOPEC in the Fushan Sag in the northern part of Hainan Island. For the first time, a set of granite interlayers between the Paleogene and Cretaceous was discovered in the northern part of Hainan Island, and systematic petrological, geochronological, and structural geological analyses were conducted on granite core samples. Results:The research results indicate that the granite interlayers in this set are mainly composed of plagioclase granite, diorite granite, and granodiorite, which undergo mineralization and alteration; The zircon U- Pb results range from 260.7 ~ 250.0 Ma, indicating that the crystallization age of this set of granite rocks is from the Middle- Permian to the Early- Triassic; Combining the sedimentary ages of the Changliu Formation of the Paleogene and the Baowan Formation of the Cretaceous, this study limits the emplacement time of the granite rock set to between the middle—late Late Cretaceous and the Paleocene. Conclusions:Based on regional geological surveys, oil and gas drilling, geophysical data, and previous research results, this study identified a late Cretaceous NW—SE trending compressional structure in the northern part of Hainan Island, providing new evidence for the study of the Late Mesozoic extensional compressional structure transition in South China.