A comparison with ground transient electromagnetic (EM) method shows that airborne transient EM is more affected by noise, such as sefrics and motion noise. And the influence of background field noise couldn't be neglected. The background contains primary field information, which causes the amplitude of background field to be much larger than the secondary field signal containing geological information. In order to obtain the underground electrical structure clearly, it is necessary to find an efficient method to remove background field. In this paper, the background field problem is studied by linear interpolation. First, according to the responses of the front and back line obtained from the high flight, after cumulative and average calculation, the responses of the same time channel of each measured line in a cycle can be obtained as the background field responses of the corresponding time channel of each line. Using interpolation principle, the background field above each measuring line in the working area is calculated by flight time, so as to remove the background field. Through the real data removal, this method can better remove the background field and then show the anomaly, and can clearly determine the location of the anomaly after removal. Time-domain AEM data preprocessing contains many processing modules. The linear interpolation method proposed in this paper lays a foundation for efficient data preprocessing.