Plasmonic demultiplexers hold promise for the realization of the subwavelength and high-splitting ratio dichroic splitter and have a wide range of applications from optical communication, and manipulation to ultrafast data treatment. However, this vision has not been realized for a long time due to lacking the suitable splitting structure design, which limits its further development of integrated photonic circuits. Here, we demonstrate a plasmonic demultiplexer with subwavelength feature size (0.54 µm) and broadband spectral (620-870 nm) range, and high-splitting ratio (17 dB in experiments and 20 dB in calculations). It consists of two adjacent Fabry-Perot cavities (covered by PMMA polymer) and coupling gratings, which are integrated with the Au waveguide. The relatively simple double cavities design of our device has a simple theoretical analysis and fabrication process. Our work has relevance for various optical applications, such as multiple wavelength photodetectors and optical multichannel interconnects.