The effect of La, Gd, Er and Lu dopings (0.2 per unit cell) on the electronic, magnetic and optical properties of BiFe0.9Co0.1O3 (BFCO) were investigated by first-principles calculations. It is found that the La- and Er-dopings reduce the band gaps to 1.72 and 0.81 eV and convert the systems to half-metallic, while the Lu and Gd dopings convert the systems to metallic. Notably, the rare-earth element dopings can increase the total magnetic moments by ∼0.2 μB per cell. The static dielectric function is equal to 9.0, 7.0, 7.4, 9.2 and 10.5 for the pristine Er-, La-, Gd- and Lu-doped BFCOs respectively. Moreover, the dopings significantly increase the optical reflectivity and the reflection index. Our results show that electronic, magnetic and optical properties in BFCO could be effectively modulated by rare-earth element dopings.