AbstractIn view of the existing situation of low level of the 1‐D interpretation of the Transient Electromagnetics (TEM) in field work, we start from the central loop method, which is widely used in field work, to make improvement. Based on the principle of superposition of the Electromagnetics (EM), we deduce the secondary field (pure abnormal field) formulas of the 2.5‐dimensional finite element method (FEM) from the Maxwell's equations. This algorithm uses triangle FEM grids, which can not only fit the geo‐electric media well but also decrease the number of nodes and cells. It adopts pivoting LU decomposition to solve the linear equations, which needs only to change the right hand items and perform an iteration when increasing a measuring point. A new scheme is used to choose wavenumbers, which lets the wavenumber m glide with given computational time t. Finally we give examples to show that the forward modeling code calculates directly the secondary field and makes the computation faster with higher calculating accuracy.