Objective To evaluate the effects of incision width on visual outcomes by comparing visual acuity, refractive error, higher-order aberrations and corneal topography parameters after femtosecond small-incision lenticule extraction (SMILE). Methods This was a prospective case-control study. Thirty-four patients (68 eyes) who underwent SMILE to correct myopia and myopic astigmatism were included. We divided them into two groups by a side cut of 2 mm or 4 mm. Twenty-two patients (44 eyes) were in the 2 mm side-cut group while 12 patients (24 eyes) were in the 4 mm side-cut group. All patients underwent routine examinations before the surgery in case of surgical contraindications. Preoperatively and 1 week and 1, 3 and 6 months postoperatively, we measured UCVA, BCVA, refractive error, total higher-order aberrations (HOAs), spherical aberrations (Z40), vertical coma (Z3-1), horizontal coma (Z31), anterior corneal surface astigmatism, corneal irregularity measurement (CIM) and shape factor (SF). A repeated measures analysis of variance was used to compare the differences in these data between the two groups and between pre- and post-operation. Results No differences were found in UCVA, BCVA or refractive error between the two groups at any time point. For Z3-1, the outcomes in the 4 mm side-cut group were higher than in the 2 mm side-cut group at 1 week postoperatively (t=－2.031, P＜0.05). For anterior corneal surface astigmatism, the outcomes in the 4 mm side-cut group were higher than in the 2 mm side-cut group at 1 week and 1 month postoperatively (t=－2.126, －2.108, P＜0.05). For CIM, the outcomes in the 4 mm side-cut group were higher than in the 2 mm side-cut group at 1 week and 1 month postoperatively (t=－2.360, －2.395, P＜0.05). Conclusion When considering Z3-1, anterior corneal surface astigmatism and CIM, SMILE with a 2 mm side cut has advantages over SMILE with a 4 mm side cut.