Abstract: Inverse distance weighting (IDW) is a simple and practical interpolation method. Taking the global ionospheric map (GIM) product as a sample, the latitude and longitude anisotropy of the ionospheric total electron content (TEC) distribution is taken into account with an introduction of the latitude and longitude adjustment factor r. Six ionospheric distance calculation schemes are used for testing and results show that the correlation of the ionospheric TEC with the longitude direction is higher than the latitude direction and different ionospheric distance calculation schemes can effectively improve the IDW interpolation accuracy. Using the IDW interpolation based on an optimal scheme, a long-term interpolation accuracy is analyzed and results show that the precision of the optimal scheme interpolation is about 25% higher than the general IDW interpolation precision in the ionospheric activity area (20° north and south latitude) for GIM data before and after spring vernal and summer solstice, autumnal equinox and winter solstice in 12 consecutive years. When the ionospheric activity was active at 3-5 hours after 14 o'clock in the local time of 2014, the interpolation accuracy base on the optimal scheme was improved obviously with an interpolation error smaller than 4.0 TECU.