In recent years, with the development of related theories of wire gratings and surface plasmons, it has become a trend to study gratings based on surface plasmonic wire gratings. Most of today's devices are pursuing miniaturization and high integration. Surface plasmons have made great contributions to the research and development of grating-based devices by virtue of their unique properties. Based on the finite element method we studied the surface plasmonic wire gratings. The finite element simulation model and analysis of plane electromagnetic wave incident by line grating on dielectric substrate are carried out by COMSOL software. The refraction, transmission and reflection coefficients of TE wave and TM wave of transverse electromagnetic wave, specular reflection and first-order diffraction are calculated. The results show that the sum of all coefficients is always less than 1. The value of mirror reflection coefficient increases relatively steadily with the incident angle of TE wave, which is more obvious when TM wave is incident. About half of the wave is absorbed by line. Another important feature of TM wave is that mirror reflection hardly occurs near 50-60 degrees.