Multiferroic functional materials are of significant importance in modern daily life and industry. Here we report a multiferroic functional material Bi0.85Gd0.15Fe1-x CoxO3(x=0, 0.04, 0.08, 0.12) thin film which was synthesized on Pt/Ti/SiO2/Si and ITO substrates by sol-gel process, involving ferroic nitrate, bismuth nitrate, gadolinium nitrate, cobalt nitrate as raw materials, ethylene glycol methyl ether as solvent, and citric acid as chelating agent forming precursor solution. A comprehensive investigation of structure, ferroelectric property, magnetic property and optical band gap of Gd3+ and Co3+co-doping BiFeO3 was conducted. XRD results reveal that all films showing a rhombohedral structure in the (111) direction, and SEM results exhibit that co-doping could refine the grains. According to the results of ferroelectricity and leakage current test analysis, the maximum residual polarization value reaches 2Pr =15.71 μC/cm2 when the Co3+ doping amount is 8%, at which amount also achieves the maximum saturation magnetization 37.78 emu/cm3. The leakage current conduction mechanism of all samples is ohmic conduction mechanism. Last but not least, a modulated bandgap effect according to the absorption spectrum and the fitting results of the Tauc formula were observed. In the co-doping film, the optical bandgap gradually decreases with the increase of the doping amount. In particular, when the Co3+doping amount is 12%, the optical bandgap decreases to 1.96 eV.