A compact and broadband polarization beam splitter (PBS) based on silicon (Si) nitride (SiN)-on-Si-on-insulator multilayer platform with vertical asymmetrical directional coupler (ADC) is designed and analyzed. The vertical ADC is consisted of two waveguides, which are placed in the bottom Si layer and upper SiN layer separately. It is found that by properly choosing the values of the structure parameters, especially taking advantages of the extra freedom of the relative location between these two waveguides, the performance of the PBS can be significantly improved. By incorporating tapered structures into the coupling region in the further step, bandwidth of 294 nm, which is determined by the insertion loss of <1 dB and the extinction ratio of >20 dB in both polarizations, is realized within a coupling length as compact as 6.9 μm. The proposed device has a good potential to be applied in three-dimensional photonic integration, where higher integration density or more on-chip functions can be realized.