An ultrawideband polarization rotator (PR) with low insertion loss (IL) based on silicon on insulator (SOI) platform is proposed. The input TM₀  /  TE₀-polarized modes can be rotated 90 deg simultaneous using a 45-deg W-slot dual-stair waveguide with varied width. Simulation results show that with a PR in length of 4.3  μm, the IL is below 0.4 (0.42) dB and polarization extinction ratio (PER) is over 17 (16) dB for TE₀-to-TM₀ (TM₀-to-TE₀) mode conversion within bandwidth of 300 nm (from 1400 to1700 nm), which covers optical fiber communication S-, C-, L-, and U-bands. In addition, the fabrication tolerance is analyzed numerically. The proposed PR has excellent performance with compact footprint and will find potential application in large-scale high-performance PICs.