Abstract: Two methods to eliminate the ionospheric delay containingf3term are given.The first method is to eliminate ionospheric delay with triple-frequency observations.The second method is directly calculate thef3term.A simplified method is proposed that a three order polynomials is used to fit the geomagnetic field intensity of a 30°×40°area,which can achieve a rather good result.By comparison,the difference of the ionospheric delay between the calculated results by the two methods is less than 0.5 mm.