GaInP2/InGaAs/Ge triple-junction solar cells have become the main energy source for space on-orbit applications. For the sake of the three composed sub-cells, including GaInP2, InGaAs and Ge sub-cell, the monolithic triple-junction solar cells can make use of solar irradiance in the wavelength range of 300 nm to 1800 nm. Before assembled into space solar arrays, each solar cell’s current-voltage curves should be measured in laboratories on earth by AM0 solar simulators, to know their key parameters especially short circuit current. Solar cell’s current-matching is crucial for assembling into arrays. While spectral responsivity is essential for spectral mismatch factors (MMFs) calculation during the current-voltage measurement. MMF corresponding to each sub-cell should be analyzed, so spectral responsivity of each sub-cell of monolithic solar cell was required to be measured out. But sub-cells are connected in series in a monolithic multijunction solar cell, and current-limiting effect makes traditional spectral responsivity measurement which is suitable for single-junction solar cells not applicable anymore. For measuring the spectral responsivity of monolithic multi-junction solar cells, optimized bias light and bias voltage are required to make the tested target sub-cell be the current-limiting one. The wavelength range and irradiance intensity of the bias light, the direction and value of the bias voltage, should be chose and adjusted appropriately during the measurement, otherwise will lead to measurement artifacts and obtain incorrect results. In this paper, combing optimization of bias light and bias voltage with monochromatic light system, we would present method and detailed procedures for measuring the spectral responsivity of monolithic GaInP2/InGaAs/Ge triple-junction space solar cells.