Inverted GaInP/GaAs/InGaAs triple-junction solar cells are fabricated by substrate lift-off and temporary bonding techniques, and their reliability is investigated. The reliability test in an environment with a temperature of 85 ℃ and a relative humidity of 85% finds that the initial photoelectric conversion efficiency of the triple-junction cells drops sharply from 31.86% to 24.84% when the damp heat test is performed for 144 h. As the test time increases, the solar cell performance is relatively stable. External quantum efficiency and electroluminescence spectroscopy tests show that the degradation of the triple-junction cell performance mainly comes from the GaInP top cell. Under the high temperature and high humidity environment, the distribution of the element concent in the AlInP window layer changes, which results in the enhanced spectral reflectance of the material in the range from 340 nm to 480 nm. In addition, the aggregation of the high-concent Al element leads to an increase in the defect density of the top cell and a decrease in the carrier's collection efficiency of the GaInP top cell, which thus limits the overall performance of triple-junction solar cells. The results of secondary ion mass spectrometry (SIMS) also visually demonstrate this phenomenon. The results of this study prove that the AlInP window layer has an important impact on the environmental stability of multi-junction solar cells.