Abstract: With its continuous application and promotion, filling technology, such as an ultra-long distance and special pipeline arrangement, faces an increasing demand, and the demand for the fluidity of filling material is also rising. Aiming at the large risk of blocking pipes and tubes in filling technology, the serious wear of pipelines, etc., and using modified magnesium slag (MMS) and fly ash (FA) to prepare high liquidity under different ratios of a new type of paste filling material (UH-MFPB), this paper probes this material’s early strength, liquidity, and rheological properties, and establishes the relationship between liquidity and rheological parameters. First, MMS and FA samples were prepared at different ratios of certain concentrations and cured for 3, 7, 28, and 56 days to measure their uniaxial compressive strength. The uniaxial compressive strength of a UH-MFPB sample increases first and then decreases with increasing FA content, and gradually increases with curing age. When FA content is 20%, the compressive strength of the sample reaches the maximum. At 3, 7, 28, and 56 days, the intensity was 1.335, 2.161, 6.759, and 12.104 MPa, respectively. Then, the slump and spread of fresh UH-MFPB slurry were measured. They increased with FA content, with a slump of 25.6–29.2 cm and a spread of 61–93.1 cm, showing good fluidity. Then, the rheological properties of fresh UH-MFPB slurry were measured in accordance with the Herschel–Bulkley model, and the relationship between shear stress and plastic viscosity and the shear rate was discussed, as well as the effect of FA content on rheological parameters and mechanisms. The shear stress is found to increase with the shear rate, and the viscosity decreases exponentially and then slowly with increasing shear rate. Rheological parameters (yield stress, plastic viscosity, and thixotropy) decrease with increasing FA content, and the slurry undergoes shear thickening when FA content reaches 20%. The Cross viscosity model was used to fit the viscosity curve of fresh slurry. Finally, the correlation between the fluidity and rheological parameters of fresh UH-MFPB slurry was discussed, and the yield stress was found to correlate negatively with the slump, plastic viscosity, and expansion, which were quadratic polynomials. Considering all factors, when FA content is between 10% and 30%, UH-MFPB slurry has ultra-high fluidity and high strength, which can play a good role in filling slurry transport.