AIM: To observe the changes of vitreous cavity length and diopter after scleral encircling (SE) produce. METHODS: This prospective study included 68 eyes of 68 non-consecutive patients with macula-off retinal detachment who were operated by SE surgery. The corneal refractive power, ocular axial length and diopter were measured by keratometer, A-mode ultrasonic meter and computed dioptometer. RESULTS: There was no significant difference in corneal refractive power among preoperative and postoperative 1, 3 and 6 mo (0.57±0.54 D at pre-surgery;0.72±0.26 D at 1mo; 0.71±0.34 D at 3mo; 0.69±0.31 D at 6mo; all P>0.05 ). Axial lengths were obviously lengthened, especially in vitreous cavity length (17.87±3.09 mm, 19.69±3.12 mm, 18.97±3.56 mm, 18.76±3.47 mm, 18.68±3.42 mm at pre-surgery, 1wk, 1, 3 and 6mo postoperatively, P <0.05) and diopter also increased at beginning and then recovered gradually. After 1 and 3 mo, axial length (vitreous cavity length) and myopia were more and in higher degree than before surgery. CONCLUSION: The change of postoperative vitreous cavity length is the main factor that results in the changes of axial length and then makes the change of diopter.