We propose and numerically investigate a novel kind of nanoscale plasmonic wavelength demultiplexing (WDM) structure based on channel drop filters in metal-insulator-metal waveguide with reflection nanocavities. By using finite-difference time-domain simulations, it is found that the transmission efficiency of the channel drop filter can be significantly enhanced by selecting the proper distance between the drop and reflection cavities. The result can be exactly analyzed by the temporal coupled-mode theory. According to this principle, a nanoscale triple-wavelength demultiplexer with high drop efficiencies is designed. The proposed structure can find more applications for the ultra-compact WDM systems in highly integrated optical circuits.