Phononic Crystals (PCs) have important potential application in engineering by the properties of bandgaps. In the paper, the bandgap characteristic of one-dimensional PCs is attributed to the results of Single-Negative (SN) properties, e.g. negative modulus or negative density. The effective parameters of the 1D PCs were predicted by the equivalent layer concept with considering viscous damping, and the results showed that during the frequency regions of the bandgaps, the negative parameters appeared. This will be a reference for the design of acoustic negative refraction metamaterials.