Knowing how much and when water was trapped within our Moon has fundamental implications on our understanding of how the Earth-Moon system formed. Water has been detected in lunar samples but its abundance, distribution and origin are debated. To address these issues, we report water concentrations and hydrogen isotope ratios of plagioclase from ferroan anorthosites, the only available lithology thought to have crystallized directly from the lunar magma ocean (LMO). Combined with literature data, δ2H values in least-degassed lunar igneous materials range from -280 to +310‰. We interpret the results by hydrogen isotope fractionation by degassing of molecular H2 in the LMO until the formation of the primary lunar crust, with magmatic δ2H value of primordial water at the beginning of LMO being about -280‰, evolving to about +310‰ at the time of ferroan anorthite crust formation. Intermediate δ2H values observed in igneous rocks could be due to either mixing of the different end members, or from mantle sources that was degassed to different degree during magma evolution. We thereby explain the wide range of hydrogen isotope ratios of lunar lithologies. In the context of this model, H2O concentration in the moon at the beginning of LMO may be as high as 2600 ppm.