Stability and efficiency are two key indicators to evaluate the development prospect of perovskite materials in solar cells (SCs). The partial substitution of Cs or Rb for Formamidinium cation (FA) is proven to significantly improve the stability of FAPbI(3). Phase compositions of Cs- and Rb-doped FAPbI(3) play a crucial role in determining the stability, which still lack a deep understanding. Besides, bandgap variations upon adding Cs or Rb content also affect the SC efficiency, awaiting a detailed discussion. This work quantitatively analyzed the phase compositions of Cs(x)FA(1-x)PbI(3) and Rb(y)FA(1-y)PbI(3). Moreover, efficiencies of the Cs(x)FA(1-x)PbI(3) and Rb(y)FA(1-y)PbI(3) SCs were simulated using experimental data. The synchrotron X-ray diffraction patterns reveal that the Cs(0.2)FA(0.8)PbI(3) sample contains the largest amount of the perovskite phase with Pm-3m symmetry, the non-perovskite phase with P63mc symmetry of FAPbI(3) is effectively suppressed and therefore the structural stability is significantly improved. However, efficiency of the Cs(0.2)FA(0.8)PbI(3) SC is slightly lower than that of the Cs(0.15)FA(0.35)PbI(3) SC, ascribed to its enlarged bandgap and consequent narrowed light absorption waveband. For the Rb(y)FA(1-y)PbI(3) samples, the Rb(0.05)FA(0.95)PbI(3) sample shows the best structural stability, and the corresponding SC achieves the highest efficiency.