Compared with the conventional organic cation lead halide perovskite, the mixed-cation lead mixed halide perovskite Cs(x)FA(1-x)Pb(I1-yBry)(3) (FA: NH2CH = NH2) possesses a much better stability thanks to the substitutions of Cs for FA, and Br for I. Moreover, its band gap, E-g, can be tuned in a wide range to meet the request of a high efficient tandem solar cell (TSC) with it as a subcell material. In this work, efficiencies for the four-terminal Cs(x)FA(1-x)Pb(I1-yBry)(3)/c-Si TSCs are theoretically investigated. Herein, E-g of Cs(x)FA(1-x)Pb(l(1-y)Br(y))(3) is adjusted approaching 1.75 eV, which is the optimal E-g of top cell material for the TSC with c-Si as the bottom cell. By means of density functional calculations, three sets of x and y for Cs(x)FA(1-x)Pb(I1-yBry)(3), which have E-g around 1.75 eV, are obtained. And their refractive indices and extinction coefficients, are also calculated. With the thickness of perovskite top cell, d(1), changing from 100 nm to 10 mu m, the highest efficiencies of the three sets of Cs(x)FA(1-x)Pb(I1-yBry)(3)/c-Si TSCs and the corresponding thicknesses of c-Si bottom cell, d(2), are studied. For the top cell, radiative recombination is considered. For the bottom cell, Auger recombination is also taken into account. Besides, the radiation coupling effect between two subcells is included. It is found that the Cs(0.11)FA(0.89)Pb(I0.56Br0.44)(3)/c-Si TSCs achieve the highest efficiency among the three sets of TSCs with the same d(1). With a 500-nm-thick Cs(0.11)FA(0.89)Pb(I0.56Br0.44)(3) top cell, the highest efficiency of the Cs(0.11)FA(0.89)Pb(I0.56Br0.44)(3)/c-Si TSC reaches 35.5% when d(2) is 160 mu m. In an extreme situation, when the thickness of the Cs(0.11)FA(0.89)Pb (I0.56Br0.44)(3) top cell is 10 mu m, the highest efficiency of the TSC is 40.0% and d(2) is 190 mu m. (C) 2017 Elsevier Ltd. All rights reserved.