Silicon on insulator (SOI) is becoming a common platform for photonic integrated circuits because of the high refractive index contrast and the compatibility with complementary metal oxide semiconductor technology．The high refractive index contrast enhances the integration capability, but it also causes serious birefringence effects．Therefore, polarization sensitivity has become a major challenge for most silicon-based optical devices．Polarization diversity schemes are often used to address the polarization issues．Polarization rotators (PRs) and polarization splitter-rotators (PSRs) are the key components in the polarization diversity circuits．To balance small size and high extinction ratio (ER) characteristics, the 29 µm × 4 µm PR and PSR with 50 dB ER are proposed via the inverse design algorithm based on the mode evolution of the bi-level taper waveguide．The simulated results of PR indicate that the insertion loss (IL) and ER for the TM₀-TE₀ modes are about 0.2 dB and 79.8 dB at 1550 nm, and the reflection losses (RLs) of TE₀ and TM₀ are about 41.5 dB and 50 dB. The simulated results of PSR indicate that the IL and ER for the TE₀-TE₀ modes are about 0.4 dB and 53.7 dB at 1550 nm, and the RLs of TE₀ and TM₀ are about 22.5 dB and 32.6 dB. In addition, the IL and ER for the TM₀-TE₀ modes are about 0.3 dB and 52.7 dB at 1550 nm, and the RLs of TE₀ and TM₀ are about 33.6 dB and 26.3 dB. The proposed devices on the SOI platform with 220 nm-thick top silicon layer and silica cladding show great integration compatibility with conventional silicon-based optical devices．This work provides a reference for solving polarization-sensitivity problems faced by silicon-based optical devices.