Abstract: To realize the application of geomagnetic navigation on the low orbit satellite, the complexity, enormous calculation, poor real time ability of traditional Gaussian spherical harmonica function should be solved before being used to calculate the geomagnetic vector. A geomagnetic field approximation function, Geomagnetic Field Approximation Functions (GFAF), is proposed to replace the spherical harmonic function. Dipole model under a pseudo-centers is utilized to replace the main geomagnetic model in GFAF method. And the coefficient of pseudo-center of different latitude-longitude grid is obtained by polynomial fit method. Finally, the geomagnetic field vector of any position can be achieved using different interpolation algorithm. A simulation test is carried out under the condition that the increment of latitude and longitude is 0.5° and the orbital height is from 300 to 500km. The results show that the navigation accuracy of GFAF method is the same to that of geomagnetic field model. However, GFAF method effectively decreases the cost of computing and and improves the calculation efficiency.