Isopotential map of cardiac are commonly used to study the spread of excitation in the heart. In this study, we propose a method for isopotential map construction used in realtime epicardial mapping. Firstly, heart contour was extracted from canine CT images and reconstructed to a three-dimensional mesh model of the heart. Secondly, OpenGL and MFC were used to display the 3D model. Then the positions of electrodes on the model were calculated semi-automatically using a central angle dividing method. When new data sampled by Epicardial Mapping System was transferred to the computer and filtered by an adaptive least mean square filter, Kriging interpolation was employed to calculate potentials at unmeasured region. The method was theoretical feasible in animal experiments. Results indicated that isopotential maps in this paper could specifically reflect the propagation process of excitement during both sinus rhythm and complex arrhythmia for instance atrial fibrillation.