Abstract: To investigate the influence of geometric characteristics (length, radius) and travel direction (upward, downward) of highway spiral tunnels on drivers’ cognitive load, a naturalistic driving experiment was conducted. Eye-tracking technology and questionnaires were used to collect drivers’ eye movement behavior indicators and subjective evaluation data, and the cognitive load of drivers in spiral tunnels with different geometric parameters was analyzed. The results show that in long-distance, small-radius spiral tunnels (e.g., Hankou Tunnel) compared with short-distance, large-radius tunnels (e.g., Nanping Tunnel), drivers’ average fixation duration increased by 12.9%, average pupil diameter expanded by 7.7%, average saccade duration prolonged by 34%, and average saccade amplitude decreased by 14%, indicating that complex geometric features significantly increase cognitive load. During upward travel in spiral tunnels, drivers' average fixation duration increased by 10.2%, average pupil diameter expanded by 4%, average saccade duration increased by 10.6%, and average saccade amplitude decreased by 5.5%, with these effects being independent of the influence of tunnel geometric parameters. When driving upward in long-distance, small-radius spiral tunnels, the comprehensive index of subjective task load evaluation was 11.3% higher, and the degree of slope-curve illusion was 40.4% higher, compared to driving downward in short-distance, large-radius spiral tunnels. It is confirmed that “long tunnel length, small radius, and upward direction” are key influencing factors of cognitive load in highway spiral tunnel driving.