This paper reports a novel (Cs/MA)3Bi2I9 double A-site cations perovskite nanostructured photoelectrode and its fabrication method, utilizing a stoichiometric dissolution of MAI, CsI, and BiI3 solutes in DMF solvent. The (Cs/MA)3Bi2I9 perovskite electrodes are then prepared on FTO glass substrates using the spin-coating technique. Furthermore, by adjusting the molar ratio x of Cs+ to the total moles of Cs+ and MA+ in precursor, the morphology and size of the (Cs/MA)3Bi2I9 perovskite nanostructures can be effectively modulated, thereby tuning their photoelectrical properties. (Cs/MA)3Bi2I9 consists of two phases: (CsMA)3Bi2I9 formed by substituting Cs+ with MA+, and (MACs)3Bi2I9 formed by substituting MA+ with Cs+. The surface photovoltage performance of the (Cs/MA)3Bi2I9 perovskites prepared at different x values surpasses that of pure MA3Bi2I9 and Cs3Bi2I9 bismuth-based perovskites. Particularly noteworthy is that at x = 0.6, the surface photovoltage performance of (Cs/MA)3Bi2I9 perovskite is nearly double that of the pure MA3Bi2I9 and Cs3Bi2I9 perovskites.