Abstract: Objective To compare the biomechanical effects of posterior and anterior lag screw and support plate in the treatment of Haraguchi type I posterior ankle fracture. Methods Clinical data of 80 patients with Haraguchi type I posterior ankle fracture admitted from June 2020 to September 2022 were retrospectively collected. According to different internal fixation methods, they were divided into posterior and anterior lag screw group (n=38, the PA group) and support plate group (n=42, the plate group). Biomechanics, follow-up and imaging indexes were compared between the two groups. Results The peak stress in the PA group under different loads was significantly higher than that in the plate group (P < 0.05). The screw/steel plate displacement in the PA group under 1000N and 1500N loads was [(0.35±0.10) vs (0.20±0.05), (0.52±0.23) vs (0.21±0.05). mm], the fracture distal displacement [(0.40±0.11) vs (0.28±0.07), (0.64±0.26) vs (0.30±0.10), mm], which were significantly greater than those in the plate group (P < 0.05). The fracture healing time in the PA group was longer than that in the plate group (P < 0.05), and the AOFAS score 3 months after surgery was lower than that in the plate group (P < 0.05). There were no significant differences in complications, complete weight-bearing time, AOFAS score and ROM at the last follow-up between the two groups (P > 0.05). There was no significant difference in fracture reduction quality between the two groups at 3 months and the last follow-up (P > 0.05). Conclusions In the treatment of Haraguchi type I posterior ankle fracture, the reduction quality and functional improvement of posterior and anterior lag screw were comparable to that of support plate, and the latter had better biomechanical effect.