A directional coupler (DC)-based polarization-independent demultiplexer filled with SiN_x is designed to separate the 1310-nm and 1550-nm optical signals. The plasma-enhanced chemical vapor deposition (PECVD) method is used to adjust the refractive index of the SiN_x material filled in the gap between the DC waveguides. As a result, the coupling length of the transverse-electric (TE) polarization mode equals that of the transverse-magnetic (TM) polarization mode at the same wavelength, and the polarization-independent function of the device is thereby fulfilled. The ratio of the coupling lengths corresponding to the two optical signals with different wavelengths is adjusted by optimizing the gap between the waveguides. When a proper value of the coupling length ratio is chosen, the two optical signals can be output from two ports, respectively, to achieve the wavelength separation function. Modeling and simulation are conducted by the three-dimensional finite-difference time-domain method to optimize the parameters of the device and analyze its performance. The results show that the proposed demultiplexer achieves a coupling region as short as 22.8 μm and an insertion loss and a crosstalk (CT) as low as 0.05 dB and -21.58 dB, respectively. Besides, the bandwidth corresponding to a CT smaller than -10 dB reaches 79 nm, and the device offers favorable tolerance on the whole. The device designed has application potential in future integrated optical circuit systems.