A polarization beam splitter (PBS) based on silica on silicon is designed by using asymmetric Mach-Zehnder interferometer (MZI). Within the wavelength range of 1535-1565 nm, the polarization extinction ratio is larger than 20 dB, and the insertion loss is larger than -0.5 dB. In addition, the finite difference-beam propagation method is adopted, the effect of errors of width/length of multimode waveguide and asymmetric arms on the performance of PBS is calculated, respectively. The simulation results indicate that, when the length errors of multimode waveguide and asymmetric arms are less than ±2 μm and ±4 μm, respectively, the extinction ratio and the insertion loss are still well. To keep the polarization extinction ratio more than 20 dB, width error of multimode waveguide should be less than ±500 nm, and width error of asymmetric arms should be less than ±4 nm (wide arm) and ±2.5 nm (narrow arm). The widths of asymmetric arms ask for high requirement in fabrication process. The thermo-optic effect should be used to regulate refractive index of the waveguide, thus compensating the phase error caused by waveguide dimensional change, so it can be used to fabricate high-performance silica on silicon PBS in the next step.