Based on the minimum utilization of storage resources and accuracy requirements of Additional Secondary Factor (ASF) database, the definition and division of the initial latitude and longitude grid, the selection of the center point of the grid and the calculation of the propagation delay value (ASF value) of random points, and the extraction and calculation of the change law of the grid width with the distance from the center point and the change law with the propagation path are adopted. The optimum latitude and longitude grid width is obtained, which is used to build the ASF data grid. The feasibility of the method is verified by simulation and measurement.