We demonstrate theoretically and experimentally the difference between the band compression mechanisms of plasmonic gratings and plasmonic heterogratings.The plasmonic grating can pick out a fixed polarization bandwidth with changing the incidence angle,while the plasmonic heterograting composed of two different nanogratings can pick out a tunable bandwidth with changing the incidence angle and the period ratio of two gratings.The range of the plasmonic resonances of the heterograting is about 30％ wider than that of a conventional grating.Thus,the plasmonic heterograting can work efficiently in a wider wavelength range.Moreover,high polarization extinction ratio is achieved in these plasmonic devices.This gives more insights into the mechanisms in plasmonic filters and is helpful to promote the actual applications.