Abstract: Magnetic design and test should be considered in the design and making of a satellite,because the magnetic torques resulting from the interaction between the magnetic properties of the satellite and the ambient magnetic field tend to disturb its attitude.This paper presents the ambient field mapping method to determine a satellite magnetic moment.Its main point is to acquire experimentally and deduce mathematically the magnetic field data of a satellite in the limited region in earth's field.In order to do that,we must overcome two main difficulties: separating permanent moment from induced moment;effectively inhibiting noise of earth's field.This method has resolved these problems by using the following techniques in a better way:1,repeating measurement at original and inclined (or inverse) position.2,obtaining the difference between the outputs from the measuring probe and the reference probe.3,let the axis of the probe direct to the earth's field east,turning the test subject over 90° of angle to keep relative magnetic state between it and the probe.The results that have been attained by analysis and test indicate that the error is not more than 10% for magnetic moment above 500 mA-m².It can be expected that the precision can be increased after improvement.