Based on the principles of transient test technique, an approximate model of lateral transient heat conduction in the sidebars of a plate-fin heat exchanger is proposed and solved, and the side-bar mass correcting factors are presented in the form of graphs modified side-bar mass is calculated from the correcting factor and is added into the solid mass of the heat transfer surface so that the available transient test techniques in which a homogeneous porous solid material assumption is made are still valid. The results obtained in the experiment show that the present approach is reasoable and simple.