The energy band structure of a lossy 2D metallic photonic crystal is solved and analysed. The transverse magnetic mode and energy band structure are obtained under the non-lossy free-electron model with the interfacial operator approach. When the Drude model with loss is considered and the homotopy method is employed to correct the eigen-frequency, the corresponding imaginary parts are obtained. The results show that the loss reaches the maximum value at the surface plasma frequency. When the frequency changes, the loss diminishes rapidly and varies with the change of reciprocal-lattice vector value. Even though the metal loss is considered, the real part of the eigen-frequency changes a little. The combination of the interfacial operator approach and the homotopy method provides an efficient technique support for the study of metallic photonic crystals.