A 2.5D finite element forward algorithm for TEM with block linear conductivity was put forward in this paper. Firstly, based on the Maxwell equations, 2-order dual differential equation along strike was obtained by Laplace and Fourier transform. Then finite element linear equation system was obtained from coupling differential equation by using the Galerkin method, and yields the numerical solution of 2.5D transient electromagnetic field. The conductivity in finite element mesh is linear but uniform. In addition, the total field is decomposed into background field and secondary field: the former is solved by analytical method, and the latter is calculated by numerical method. Finally, 2.5D transient electromagnetic response curves of several typical geoelectric section models were computed.