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In line operation, Ultra-High Voltage Direct Current (UHVDC) transmission lines must meet the corresponding electromagnetic environment requirements; the ion flow field is a crucial environmental index of ultra-high-voltage DC transmission lines. Under the combined effects of humidity, air pressure, and temperature, the ion flow field is easily influenced. A calculation method for the ion flow field of UHVDC transmission lines that considers the combined effects of humidity, air pressure, and temperature is proposed to investigate the impact on the ion flow field. Based on this, when the humidity exceeds 60%, the influence of the charging characteristics of airborne droplets on the ion flow field is considered to align the calculations more closely with actual conditions. An improved finite element method is used to calculate the ground synthetic electric field strength and ion flow density of the ±800 kV line under these combined effects, and its effectiveness is validated through empirical testing. The results show that the synthetic electric field strength under the positive line decreases with increasing humidity, peaks at 90% humidity under the negative line and reaches a minimum at 50% humidity for the negative line. When considering the charging characteristics of droplets with humidity above 60%, the maximum synthetic electric field strength under the positive line is increased by 5.46%, and under the negative line by 5.5%, while the ground ion flow density remains largely unchanged. Finally, in conjunction with the electromagnetic environmental parameter limits for UHVDC transmission lines promulgated by China, improvement measures are proposed for the ground ion flow field. • Reveals coupling of humidity, pressure, and temperature in ion flow fields. • Enhances accuracy of ground ion flow predictions under varied conditions. • Accurately predicts ion flow under various natural conditions. • Basis for optimizing UHVDC design, reducing costs, and enhancing economic efficiency. [ABSTRACT FROM AUTHOR]