Abstract: The traditional Cr-Mo gas cylinder steel production exhibits a significant strength-toughness inversion challenge, and current research both domestically and internationally indicates that its strength level is generally below 1 200 MPa, failing to meet the future requirements for lightweight and safety of high-pressure gas cylinders. In this study, by adding an appropriate amount of Ni element and small amounts of Nb and V elements to the traditional Cr-Mo seamless gas cylinder steel, the chemical composition (mass fraction) was designed as follows: w(C)=0.25%, w(Si)=0.35%, w(Mn)=1.0%, w(Cr)=1.0%, w(Mo)=0.8%, w(Nb)=0.04%, w(V)=0.20%, w(Ni)=1.5% for high-pressure seamless gas cylinder steel. After 8-pass hot rolling in the complete austenitization temperature range, the steel was directly water-quenched to room temperature. Two heat treatment processes, namely direct tempering and 870 ℃ quenching + tempering, were designed to investigate the microstructure and mechanical properties of the steel plates under different tempering processes. The results show that under the direct tempering process, the microstructure of the steel plate consists of flattened grains. After the 870 ℃ quenching process, uniform and fine equiaxed grains are obtained, with significant grain refinement from 5.4 μm to 2.6 μm, a refinement of approximately 51%. As the tempering temperature increases, the dislocation density gradually decreases. Under the direct tempering process, the dislocation density of the steel plate decreases from 6.511×10¹⁵ m^-2 to 2.019×10¹⁵ m^-2, with a faster dislocation recovery rate. When the 870 ℃ quenching + tempering process is adopted, the comprehensive mechanical properties of the steel plate are significantly better than those under the direct tempering process, and the transverse impact toughness at -50 ℃ is markedly improved. Particularly, under the process of quenching at 870 ℃ for 1 h + tempering at 590 ℃ for 2 h, the steel plate exhibits the best performance, with a tensile strength of 1 361 MPa, yield strength of 1 310 MPa, elongation of 12%. The transverse impact toughness is 90.1 J/cm²at room temperature and 86.3 J/cm² at -50 ℃ ylinders.