In this paper, it is investigated experimentally that the open-circuit voltage of InGaP/InGaAs/Ge triple-junction solar cell is influenced by the vibration direction of the electric vector of linearly polarized light. The results show that those voltages are subjected to periodic changes with the vibration direction of the electric vector, and the magnitude of change is about 1%4%. It is due to the effect of anisotropy of band structure in the three-junction solar cell,which is confirmed by theoretical analysis. In addition, through combining the experimental study with theoretical analysis, the relationship between open-circuit voltage and illumination, is studied, showing that they are related to each other logarithmically, which is different from characteristics of a single-junction solar cell. From fitting results, it is indicated that the triple-junction solar cells may be regarded as three diodes connected in series, of which the ideality factor is close to 6. This is due to many defects in three-junction solar cells.