Photoluminescence technique are used to diagnose the quality of quantum wells. The influences on the fluorescence spectra of quantum wells due to thickness fluctuations of quantum wells, fluctuation of aluminium content, various defects, and unintentional impurities, are discussed. And inversely, the possible reasons causing degradation of quantum wells are deduced from the photoluminescence spectra. To some extent the diagnosis can provide certain basic information for improving the molecular beam epitaxy technology.