This study aims to propose a nanodisk structure embedding a rectangular metal block. The Fabry-Perot cavity formed by this structure is used to enhance the coupling effect of the surface plasmons. The structure has a narrow bandwidth, high quality factor, and high filtering performance. Herein, a multi-channel wavelength-division multiplexer is constructed by multiple cavities coupling. The influence of the horizontal and vertical widths of the rectangular metal block and the coupling distances between the embedded disk and rectangular metal block on the transmission characteristics of the device is described with the time-domain finite-difference method, for which a device without embedded rectangular metal block is used as the control group. A multi-channel wavelength division multiplexer is realized according to its transmission characteristics. The filter shows strong transmission characteristics when the disk resonance filter is embedded in the rectangular metal block; its full width at half maximum is significantly reduced and the quality factor is increased. By coupling a number of inline rectangular-metal-block/disk resonators, we construct the filter. Such plasmon multi-channel wavelength-division multiplexers can provide two- and three-channel demultiplexing functions. The resonant wavelength of each channel can be adjusted by selection of the parameters of the embedded metal block in the resonator, the transmission efficiency can reach up to 70%, and the minimum insertion loss is 1.549 dB. The average operating range is 189 nm, and there is no adjacent-channel crosstalk. We demonstrate that the proposed structure has good de-multiplexing frequency characteristics.