In order to study the relationship between the output frequency of Smith-Purcell free-electron laser and the depth, length and width of grating groove, the rectangular grating size of Smith-Purcell free-electron laser is discussed by means of theoretical analysis and particle simulation methods. First, by particle simulation, we found that the output frequency of Smith-Purcell free-electron laser decreases with the increase of grating groove depth, grating groove length and grating groove width. Then, the grating groove of Smith-Purcell free-electron laser is discussed by means of theoretical analysis. It is found that each grating groove can be equivalent to an LC resonant circuit. There are two kinds of radiation in Smith-Purcell free-electron laser, one is Smith-Purcell radiation and the other is LC oscillatory radiation. Finally, the LC oscillatory radiation of grating groove is estimated as well. It is found that the simulated values of output frequency of Smith-Purcell free-electron laser and the estimation values of LC oscillatory radiation of grating groove are both on the order of 102 GHz, and the variation is consistent. It is inferred that the output frequency of Smith-Purcell free-electron laser should be determined by grating groove rather than resonator.