Two kinds of ultra-broadband single polarization single mode (SPSM) square-lattice photonic crystal fibers with liquid crystal infiltrating are designed based on the mode cut-off method. The effects of the additional holes, the pitches in the fiber core, and the diameters of the liquid crystal filled holes on the SPSM operation are investigated with the full vector finite element method. The designed band-tunable ultra-broadband SPSM single-core photonic crystal fiber is obtained. Its SPSM operation can be tuned from 0.75 μm to 2.59 μm, and the widest SPSM bandwidth is 970 nm under a confinement loss of less than 0.1 dB·km -1. Moreover, the designed dual-core photonic crystal fiber is achieved with SPSM operation within the wavelength range from 0.98 μm to 1.74 μm, which can be applied in the wavelength division demultiplexer. In addition, a photonic crystal fiber wavelength splitter with a length of only 1.06 mm is fabricated, which can be used to realize the splitting of 1.31 μm and 1.55 μm lights with SPSM operation.