Background: To compare the accuracy of intraocular lens (IOL) calculation formulas in cataract patients with high myopia using the measurements of a swept-source optical coherence tomography (SS-OCT) biometer, the IOLMaster700.Methods: Patients with axial length (AL) equal to or longer than 26.00mm undergoing uneventful cataract surgery were enrolled. Kane, Hill-RBF3.0, EVO, Barrett Universal II, Haigis, and SRK/T formulas were evaluated with the measurements taken by IOLMaster700. The manifest refraction was measured at one month postoperatively. After the mean refractive errors were zeroed out, the mean absolute error (MAE), the median absolute error (MedAE), the standard deviation of prediction error (SD), and the percentage of eyes with refractive errors within ±0.25, ±0.50, ±0.75, and ±1.00 diopter (D) were calculated. A subgroup analysis was based on the axial length.Results: 65 eyes of 65 patients were included. There were significant differences between the absolute refractive errors predicted by these formulas (P<0.05), but no significant differences between the percentage of eyes within a certain range of refractive errors (P>0.05). The Kane formula achieved the lowest MAE (0.323D), SD (0.402D), followed by EVO, Hill-RBF3.0, Barrett Universal II, Haigis, and SRK/T formulas. The Kane formula also had the highest percentage of eyes with refractive errors within ±0.50D (80.0%) and ±1.00D (98.5%). The Hill-RBF3.0 formula achieved the lowest MedAE (0.240D). In eyes with an AL ≥ 30.00mm, the Kane formula had the lowest MAE (0.358D).Conclusion: Newer formulas such as Kane, EVO, Hill-RBF3.0, and Barrett Universal II show the highest accuracy in refractive prediction in eyes with high myopia while using the measurements of IOLMaster700. In extremely myopic eyes with an AL ≥ 30.00mm, the Kane formula is the most accurate.