An ultra-compact 1×2 demultiplexer based on directional coupler (DC) waveguide is proposed to separate the 1310 nm wavelength from 1550 nm wavelength, in which a new Si ₃N ₄/SiN _x/Si ₃N ₄sandwiched structure is used to realize polarization insensitivity. Firstly, the new sandwiched structure is designed to be polarization-independent. The coupling lengths of two orthogonal polarization modes at the same wavelength versus the gap between two parallel SiN _xwaveguides g ₁are calculated with several groups of structure parameters of the demultiplexer. The result shows that the coupling lengths for the two orthogonal polarization modes at the same wavelength can be identical by choosing the proper g ₁. Then, how to realize the function of wavelength separation is studied. When one wavelength propagates at even multiple of coupling length and the other wavelength propagates at odd multiple of coupling length, and vice versa, the two working wavelengths will output from different output ports, thereby the two wavelengths are successfully separated. Under the premise of satisfying such conditions, a comparison of size and performance among the devices with different groups of structure parameters is given to find the best one. The demultiplexer based on Si ₃N ₄/SiO ₂platform has a compact structure, easy integration and good tolerance. Three-dimensional(3D) finite-difference time-domain method is used for simulation, and the results show that the length of the DC waveguide is only 23 μm; the insertion loss and crosstalk are as low as 0.1 dB and–26.23 dB respectively; a broad 3-dB bandwidth of 200 nm is achieved. To demonstrate the transmission characteristics of the demultiplexer, the evolution of the excited fundamental mode in the demultiplexer is also given. The novel demultiplexer is polarization-independent and can work at 1310 nm and 1550 nm wavelengths simultaneously. It has a potential application value in future integrated optical circuits.