Objective To analyze the interactive effects of defocus and spherical aberration on the visual acuity of human eyes， both individually and as a group. Methods Ten volunteer subjects were enrolled in this experimental study. After treatment to induce mydriasis and ciliary paralysis in a dark room， the subjects were tested for visual acuity at 100% contrast using a self-designed， adaptive， high-order aberration and visual analysis system. The visual acuity test was performed by adding different levels and directions of defocus after adjusting for the inherent aberration of each eye. Furthermore， we tested visual acuity by adding +/-0.1 µm of spherical aberration in combination with different levels of defocus. Using a single-element linear regression analysis for visual accuracy with the add-ons for aberration changes， we also obtained the representative binomial fitting equation. Results Visual acuity decreased with an increasing level of added defocus. However， visual acuity improved when the defocus was added in combination with spherical aberrations in the same direction. Based on our data， visual acuity reached a maximum value with any of the following conditions： （1）spherical aberration=0 and defocus=+0.1 µm， （2）spherical aberration=+0.1 µm and defocus=+0.3~+0.5 µm， （3）spherical aberration=-0.1 µm and defocus=0 µm. Conclusion Spherical aberration and defocus reciprocally affect visual acuity. Spherical aberration in the same direction contributes to the tolerance of defocus. The co-presence of defocus and spherical aberration in the same direction produce less interference on visual acuity because of the counteractive effect between spherical aberration and defocus.