Based on the triangular mesh dissection of the finite element method, this paper presents the 2.5-D airborne transient electromagnetic method forward modeling. Firstly the time domain electromagnetic partial equations are converted into laplace domain with the numerical time-frequency transform algorithm, then the 2.5-D expressions are obtained from 3-D case by adopting the fourier transform. The airborne data are acquired from the results of the digital inverse laplace transform of the 2-D electric and magnetic components in laplacian domain, which is computed using finite element method.Instead of the conventional algorithm, the anomalous field method is employed throughout the finite element process, so that the source singularity of total field is avoided. the source impact is embodied in the differencial equation by appling the background electromagnetic field item.The computing accuracy and efficiency would be managed rigorously due to the wide dynamic range of the transient singnal, as well as the two-time inverse laplace and fourier transforms. Otherwise the accumulative error will soar acceptably. The synthetic model experiments shows that the numeric modeling solutions match the analytical results of homogeneous and layered erath responses well, which also proves the effectiveness of the algorithm.