Abstract: Two types of A100 steel pre-alloyed powders with different particle sizes (1# small particle size powders and 2# large particle size powders) were selected, the styrene-butadiene rubber used as the forming agent was added, and the unidirectional pressing was carried out. The effects of styrene-butadiene rubber content and pressing pressure on the compact density were studied. The high-density A100 high alloy steels were prepared by vacuum pressureless sintering with the constant-temperature and variable-temperature supersolidus liquid-phase sintering (SLPS) process. The effects of powder particle size, pressing, and sintering process parameters on the relative density, microstructure, and mechanical properties of the powder metallurgy A100 steels were studied by metallographic microscopy, scanning electron microscopy, and tensile test, and the sintering densification mechanism was analyzed. The results show that, the larger the powder particle size, the lower the required forming agent content, and the better the compression performance. The optimal forming agent mass fraction for 1# powders and 2# powders is 0.7% and 0.5%, respectively, the optimal pressing pressure is 710 MPa and 800 MPa, respectively, and the highest green density obtained is 5.751 g·cm⁻³ and 5.890 g·cm⁻³, respectively. The smaller the particle size, the better the sintering performance. The 1# green compact is sintered by constant-temperature SLPS at 1430 ℃ for 90 min, resulting in a relative density of 95.0%. The 1# green compact is also sintered by variable-temperature SLPS, with a 10-min holding at 1440 ℃ followed by a 72-min sintering at 1430 ℃. The relative density of the sintered samples reaches 99.4%, the tensile strength is 1734 MPa, the yield strength is 1286 MPa, the elongation is 5.2%, and the fracture toughness is 81.7 MPa·m^1/2. High-performance A100 steel powder metallurgy materials are effectively obtained with the method of powder compaction and sintering.