We have quantitatively investigated the two-step photon absorption in quantum dot solar cells (QDSCs) by using absolute intensity calibrated photoluminescence (PL) spectroscopy. Multi-stacked InAs/AlGaAs QDs were fabricated in the i-region of p-i-n single-junction solar cells by molecular beam epitaxy. Hyperspectral imaging, which combines both the spatial and spectral dimensions of the luminescence, was used to investigate QD ground state PL at room temperature. Two lasers simultaneously excited the QDSCs to characterize two-step photon absorption. An excitation laser caused interband transition to generate photo-carriers in QDs, and the other infrared (IR) laser excited intraband transition from the QD states. As the result of two-step photon absorption, reduction in PL intensity was clearly observed under IR bias excitation. We compared absolute PL intensity with and without IR illumination, and obtain quasi-Fermi level splitting and two-step photon absorption efficiency in QDSCs under study. Compared with the photocurrent measurements, PL spectroscopy performed under open-circuit conditions, so that higher carrier filling ratio can be realized in QDs. Furthermore, PL can characterize fundamental transition on two-step photon absorption because photocurrent production needs carrier extraction to the external circuit. Quantitative analysis of two-step photon absorption by PL spectroscopy could clarify physical insights, and it would be beneficial to realize high efficiency intermediate band solar cells.