We proposed a polarization-insensitive 1×3 optical power splitter with broadband performance. Ion assisted deposition was used to change the refractive index of the core SiN_x in a sandwiched structure, so that the beat lengths of the orthogonal polarization modes were equal and thus polarization insensitivity was achieved. A combined structure of trapezoidal multimode interference waveguide and tapered waveguide was introduced to realize broad bandwidth, low loss, and good beam splitting uniformity. Furthermore, a finite-difference time-domain method was applied to simulation and optimization, and the results demonstrate that the length of the multimode interference waveguide is only 13.2 μm; the excess loss and imbalance of the splitter are lower than 0.07 dB and 0.03 dB, respectively. In addition, the 0.5 dB-loss bandwidth is as high as 255 nm, which can cover S, C, L, U, and part of E bands. Therefore, the proposed splitter has potential application value in the future integrated optical systems.