Light harvesting is the first step of photovoltaic process in polymer solar cells. However, such donor: acceptor bulk junction layers are usually featured with vertical phase segregation as well as film-depth-dependent molecular aggregation, chain orientation and crystallinity, leading to a significant variation of photon absorption and exciton generation at different film-depths. We propose an experimentally and numerically accessible method to investigate the depth-dependent light harvesting behaviors in the active layer in polymer solar cells. A low-pressure oxygen plasma is utilized to etch the active layer gradually which is monitored by a light absorption spectrometer. Including the obtained sublayer absorption spectra into transfer matrix optical model yields depth-dependent optical properties and exciton generation profiles, which contribute to quantum efficiency and short-circuit current. This approach is helpful to optimize vertical material variation and provide insights into photovoltaic process.