In this paper, we present a proposal for compact photonic wavelength filtering and 3-dB wavelength splitting device based on nanoplasmonic metal-insulator-metal structure. The working performance of the device has been accurately simulated using the temporal coupled-mode theory. We use a numerical simulation method of eigenmode expansion propagation for the overall design process. We show that the transmission efficiency of the drop filter can be significantly enhanced by applying specifically optimization of nanostub waveguide. The proposed structure has potential applications in highly efficient ultra-compact integration circuits as well as in optical communication systems at nanoscale.