Abstract: This paper first applies a space element of Finite Element Method of Lines to analyze three dimensional thermal fields in a steady state. Using a rectangular strip element as an example, the theoretical basis of a three dimensional space element is described. Meanwhile, the rectangular strip elements' Ordinary Differential Equations and boundary conditions are deduced by the principle of minimum potential energy. In addition, the software based on the thermal field of Finite Element Method of Line is introduced. Finally, the comparison of numeric results and analytic solution shows the effectiveness of the method.