Abstract: A great breakthrough has recently been made for oil and gas exploration in the weathering crust reservoir of granite buried hill on the Songnan low uplift in the deep-water areas of the Qiongdongnan Basin. One of the difficulties in seismic exploration for granite weathering crust reservoirs is the determination of reservoir bottom interface. Taking the weathering crust of pre-Paleogene granite in the deep water of the Qiongdongnan Basin as an object of study, and based on the integration of drilling wells, well logging and seismic data, this paper established a calibration model for determination of the bottom interface of the granite weathering crust reservoir. Forward seismic simulation method is adopted to define the relationship between the weathering crust reservoirs of granite with different thickness and seismic responses. Started from the study of the relationship between exposure time and weathering degree of buried hill, the palaeogeomorphology of buried hill of the granite is described with the impression method. On the palaeogeomorphological map, different exposure points are selected to calculate the two-way time interval for the corresponding weathering crust reservoir bottom interface relative to the top interface of the seismic profiles. Based on forward simulation, multi-point fitting analysis is carried out to gain the corresponding quantitative relationship. Finally, taking the top interface of the weathering crust reservoir as a reference, the bottom interface of the weathering crust reservoir and the quantitative time-thickness relationship of the weathering crust reservoir are obtained. Using the above method to calibrate the bottom interface of the buried hill reservoir, the prediction accuracy of the weathering crust reservoir of the granite buried hill will be highly improved.