A compact polarization splitter (PS) based on tapered cascaded multimode interference (MMI) couplers in slot waveguide structures was proposed and characterized. By utilizing two cascaded MMI couplers, the two orthogonal polarizations, quasi-TE mode and quasi-TM mode, are efficiently separated without the necessary that the total length of the MMI must be common multiple of the self-imaging lengths of the two polarizations. This greatly reduces the length of the PS and improves the design flexibility. Meanwhile, the high birefringence of slot waveguides and the tapered MMI structure can further shorten the device length. Results show that a PS with a length of 42 μm in the MMI sections can be achieved, where the polarization extinction ratios (ERs) for the quasi-TE and quasi-TM modes are 29.8 and 31.4 dB, respectively. Optical bandwidth of 43 nm is obtained for keeping the ER over 20 dB. Moreover, fabrication tolerances to the structural parameters were investigated, and field evolution along the propagation distance through the PS was also demonstrated.