We introduce the concept of the photonic crystal (PC) into the waveguide system and propose a metal–dielectric–metal (MDM) waveguide system coupled with three periodic stubs and a multimode cavity. This system can realize multiple ultra-narrow plasmon-induced transparency (PIT) effects with a minimum linewidth of about 4 nm. These PIT peaks exhibit tunable evolution characteristics of two groups. The wavelengths and linewidths of one group of PIT peaks in the transmission spectra only can be changed slightly, while the wavelengths of another group of PIT peaks can be changed significantly by adjusting one of the geometric parameters of the cavity. This research may open up new design ideas for ultra-narrowband multichannel filters and switches.