The coexistence of luminescent and magnetic properties in nanocomposites has attracted the interest of many researchers to study nanocomposites due to their unique properties. In this study, we present a convenient synthesis of Bi₆Fe_1.9Co_0.1Ti₃O₁₈/SiO₂/NaGdF₄:Yb³⁺,Er³⁺ (BFCTO/SiO₂/NGF) core/shell/satellite nanocomposites with precise control over the thickness of the SiO₂ shell. By adjusting the amount of tetraethyl orthosilicate (TEOS), we obtained BFCTO/SiO₂/NGF samples with different SiO₂ thicknesses (0 nm, 8 nm, 13 nm, 18 nm, 25 nm and 36 nm). On account of the SiO₂ spacer effectively reducing the absorption intensity around the green and red regions, the optimal upconversion luminescence intensity of BFCTO/SiO₂/NGF was observed at a separation distance of 36 nm. Moreover, at room temperature, the hysteresis loop of BFCTO/∼13 nm SiO₂/NGF obtained the maximum remnant magnetization (2Mr) of 0.13 emu g⁻¹. Also, the BFCTO/∼36 nm SiO₂/NGF sample reached the maximum coercive field (2Hc) of 1200 Oe. Besides, the BFCTO/∼8 nm SiO₂/NGF sample had the best ferromagnetism with a saturated magnetization Ms of 0.63 emu g⁻¹.