A theoretical model of high frequency characteristics of staggered grating slow-wave structure at G band frequency is presented based on the eigen function method. Considering the convergence of spatial harmonic series, the loss parameters well as dispersion curve and coupling impedance are then derived and verified by simulation software. The results of the theoretical model match well with those from simulation, the error of dispersion characteristics is less than 0.1%, and the errors of coupling impedance and loss are both less than 10%, which proves the reliability of the theoretical model. It shows that the model is universal for structures with arbitrary staggered distance or different grating height and can be converted to double grating or rectangular waveguide grating in two extreme cases. The loss mainly occurred on the beam tunnel region of the structure and there is less loss on the top and bottom of slot area.