Abstract: During the reorganization of historical magnetic survey data for the South China Sea, it was found that the processed magnetic survey data of a specific area in the southern South China Sea exhibited low accuracy, affecting the effectiveness of data use. Data analysis reveals that the low accuracy was primarily caused by the significant errors in diurnal correction. Therefore, it is necessary to modify the diurnal correction method and reprocess the data. Based on the recollected data from various available geomagnetic observation stations and the variations of the geomagnetic field, this study simulated the magnetic diurnal variations in the study area during the survey period using three algorithms: the CM4 model, regression analysis, and regression analysis based on the data sample control of the CM4 model. The historical magnetic survey data were recorrected for diurnal variations, with data affected by magnetic disturbances removed based on geomagnetic indices, significantly enhancing data accuracy. This study addresses the problem of low accuracy in historical magnetic survey data in the study area, providing a novel approach for the fine-scale reprocessing of magnetic survey data related to the South China Sea.