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We propose and demonstrate an ultra-broadband polarization splitter-rotator (PSR) on the Silicon-On-Insulator (SOI) platform. The proposed PSR consists of a straight multi-mode waveguide, an asymmetrical directional coupler and a bent directional coupler. The multi-mode waveguide enables highly-efficient TM_0-TE_1 polarization rotation. The excited TE_1 mode is then converted to be TE_0 mode by the asymmetrical directional coupler. The remained TM_0 mode is filtered out by the bent directional coupler. On the other hand, the incident light of TE_0 mode goes through the PSR with negligible conversion and coupling. Only one-step etching is required for the proposed PSR. The fabricated PSR shows a high extinction ratio > 30.82 dB and a low loss < 0.57 dB at the central wavelength. The extinction ratio is > 20 dB over an ultra-broad wavelength band > 85 nm.