We propose a simple semi analytical model that allows to compute the transmittance and reflectance of a one dimensional subwavelength graphene strip grating under an external static magnetic field. In this model graphene is treated as an anisotropic layer with atomic thickness and a frequency dependent complex permittivity tensor. The model is based on an effective medium approach (EMA) and a rigorous phase correction. The scattering matrix approach is also used to take into account the different resonant phenomena occurring in the structure. The approach is validated against the Polynomial Modal Method (PMM) through numerical examples.