Abstract: Vertically projecting the observation point at a undulate terrain onto a horizontal plane and copying the potential at the terrain to the corresponding point at the horizontal plane,we get a fictitious potential field on a horizontal plane.An FFT is then can applied to calculate the outward normal derivativ a u／ n of the potential u on the horizontal plane.The calculated derivativeis u／ n is used approximately as the outward normal derivative of u／ n of the potential on the terrain.Based on the potential u and the normal derivativeon the terrain,Green’s formula is used to calculate the potential u at an arbitrary point above the terain.When the potential u at a level above the terrain is obtained,an FFT is used again to calculate the potential u at other level above the sources of the potentia1.The other method of continuation of potential field,e.g.,the equivalent source method, requires solving a large set of linear algebraic equations that take a hefty amount of computer memory and expends considerably the computation time.This method does not require solving algebraic equations and is of a high computation speed.The model tests demonstrated that the accuracy of this method is satisfactory.Using this method to continue the practical aeromagnetic anomaly shows a good result.