Polarization splitter-rotators (PSRs) are crucial components for controlling the polarization states of light in polarization-division multiplexing systems. However, current PSRs on the lithium niobate-on-insulator (LNOI) platform suffer from fabrication difficulty in high-precision shallow-etching and minimal feature size. In this letter, a large-bandwidth PSR is proposed based on LNOI strip-waveguide which only requires a single full etching step. The device is composed of an adiabatic taper, a Y-splitter, and a multi-mode interferometer. The minimal feature size is larger than 500 nm, which greatly reduces the difficulty of etching. Experimental results show that the insertion loss of such PSR is less than 1.5 dB with a polarization extinction ratio (PER) of more than 20 dB in a wide wavelength range of 1514–1640 nm for the TE0 mode. Additionally, a PER of 10 dB over 47 nm bandwidth (1502-1549 nm) for the TM0 mode is also demonstrated with our device. This LNOI-based PSR is promising in on-chip polarization-division multiplexing optical transmissions.