Using FDTD method, the characteristics of elastic/acoustic wave propagating in two dimensional phononic crystals (PCs) consisting of Fe square arrayed cylinders in water was investigated. The absolute band gap was found in high frequency range for Fe/water structure, and the resonance gap was found in low frequency range for coated Fe/water structure, where the Fe cylinders were softly coated by rubber. These two gaps correspond to different mechanisms. Constructed with these two kinds of PCs, a combined wide gap from high to low frequency can be achieved, so the elastic wave propagating can be prohibited in a wide frequency range. Furthermore, the combined wide band gap can be adjusted effectively through changing the structure parameters, such as the size of Fe core, thickness of coating layer, filling ratio etc.