The dual band filtering characteristics of a spoof surface plasmonic waveguide loaded with a composite slot structure are studied in the present paper. The proposed composite slot structure enables the plasmonic waveguide to work at dual bands in the microwave region, where the working bandwidth can be flexibly adjusted with a broad isolation band between them. In particular, the second bandwidth can be adjusted independently without affecting the first passband by controlling the special parameter of the composite slot. As a demonstration, the measurement results of the proposed prototype filter illustrate that the plasmonic filtering waveguide has a dual bandpass performance with excellent frequency selectivity, where the insertion loss of the dual-band filter is less than -3 dB in the frequency from 2.7 GHz to 6.7 GHz and 7.7 GHz to 9.1 GHz, and the stopband rejection level is lower than - 10 dB from 6.9 GHz to 7.5 GHz. Compared with the traditional dual band plasmonic filtering waveguide, the proposed filter has compact physical size, lower insertion loss in the passband and stronger isolation ability between the two passbands.