For exploiting phononic crystals with small periodic size and low frequency bandgap,a material ratio calculation method is proposed at fixed period with the lowest starting frequency,and then it was verificated that the equivalent density and equivalent elastic modulus in the phononic crystal bandgap calculation were correct.According to these rules we prepared phononic crystals with a periodic size of 0.04m,bandgap of 200Hz-1200Hz,and the calculated results were validated by experiments.