Motivated by the need for a more efficient simulation of material behavior at both larger length scale and longer time scale than direct molecular dynamics simulation, an atomistic field theory (AFT) for modeling and simulation of multiphase material systems has been developed. Atomistic formulation of the multiscale field theory and its corresponding finite element implementation are briefly introduced. By virtue of finite element analysis of classical continuum mechanics, we show the existing phenomena of spurious wave reflections at the interfaces between regions with different mesh sizes. AFT is employed to investigate the wave propagation in magnesium oxide from the atomistic region to the continuum region without any special numerical treatment. Unlike some other atomistic/continuum computational methods, AFT has demonstrated the capability to display both acoustic and optic types of wave motion. Numerical results show that AFT has the capability to significantly reduce the wave reflections at the interface. This work provides a more fundamental understanding of wave reflections at the atomistic/continuum interface.