The real-time attitude angles detection technology of conventional ammunition requires access to the intensity of geomagnetic field with high speed. Thus, bilinear interpolation was proposed and studied to calculate the intensity of geomagnetic field. This interpolation was premised on reducing the factors that may affect geomagnetic field. It only used longitude and latitude as parameters. In each strongest change area, three orthogonal components of geomagnetic field were computed with bilinear interpolation, errors of which were calculated and analyzed by magnetic isoline maps and statistic methods. Then the application of bilinear interpolation to attitude angles detection technology was specified. The results show that the bilinear interpolation, which has high accuracy and rapid calculation speed, could sufficiently meet the requirement of speediness and accuracy.