A dielectric cover is usually needed in waveguide slot arrays for protection or thermal insulation purposes, and it can improve the antenna performance with a proper choice of the cover parameters. In light of this, the properties of a SIW longitudinal slot with a dielectric cover were extensively investigated using equivalent relationship between SIW and conventional waveguide together with Method of Moment, as a first step to the design of a SIW slot array and its integrated radome. The accuracy and efficiency of our method was assessed against Ansoft HFSS. The computational time was significantly reduced by a one-order magnitude with acceptable computational accuracy. Finally, effects of the dielectric-cover were extensively investigated through three typical cover configurations and some instructional conclusions were drawn, which finds further application for the optimization design of the dielectric covers to improve the antenna performances.