A novel modified edge-based smoothed finite element method (modified ES-FEM) is developed to compute the band gap of acoustic metamaterials. The stiffness in the modified ES-FEM is created by the edge-based smoothed finite element method (ES-FEM) which is aimed at softening the overly stiffness of standard finite element method (FEM). On the other hand, the mass matrix is constructed by mass-redistributed method to tune the balance between the smoothed stiffness and mass matrix. The present modified ES-FEM adopts linear triangular elements generated automatically, which enables automation in computation and saving computational cost in mesh generation. Two numerical examples are presented to verify the computational efficiency of the modified ES-FEM. The numerical results have clearly demonstrated that the modified ES-FEM is very effective to minimize the dispersion errors in the simulation of band gap of acoustic metamaterials.