Objectives. To investigate whether a reduction in the O2 tension from 5 to 2% during extended culture from day 3 onwards was beneficial to human blastocyst development in vitro. Methods. We firstly identified 139 patients who had no low-quality embryos on day 3, and all the embryos were prolonged to culture on day 5 or 6. We mainly analyzed 188 patients receiving IVF/ICSI-ET for the first time, and no high-quality embryos were obtained on day 3 from January 2018 to December 2019. After transferred with one or two low-quality embryos, extended culture was performed under low O2 (5%) or ultralow O2 (2%) tension for surplus embryos. 296 embryos from 106 patients were continued to culture under 5% O2 tension, and 214 embryos from 82 patients were continued to culture under 2% O2 tension. Main outcomes compared were blastulation and high-quality blastulation rates. Results. We observed no significant differences in the blastulation and high-quality blastulation rates for high-quality cleavage-stage embryos between 2% and 5% O2 groups ( p > 0.05 ). For low-quality cleavage-stage embryos, we observed that the 2% O2 group showed a significantly higher blastulation (39.72 versus 31.08%; p = 0.043 ) rate than that in the 5% O2 group. The high-quality blastocyst formation rate (10.75 versus 8.45%; p = 0.380 ) was comparable between the 2% and 5% O2 groups. The blastulation rate reached 44.86% by culturing blastocysts an additional day under 2% O2 tension which was significantly higher than that (32.09%) under 5% O2 tension ( p < 0.05 ). Conclusion. A reduction in O2 tension from 5 to 2% after day 3 might be beneficial to the patients with no high-quality embryos. Extended culture to day 7 under 2% O2 tension increased the number of available blastocysts per IVF/ICSI cycle and was worth recommending especially for patients with few blastocysts.