In this study,a two-dimensional fluid model is proposed to simulate the underwater discharge in a He/H2O-filled bubble at atmospheric pressure.The molar fraction of water vapor is varied in the range of 0.01％-1％ to investigate the dependence of discharge dynamics and reaction products on water vapor concentration (WVC).The numerical results show that most properties of the discharge sensitively depend on the WVC.The increase of WVC leads to an increase in the electron density and discharge propagation velocity,which is attributed to Penning ionization between He* and H2O.The main positive ion switches from He+ to H2O+,while the WVC increases from 0.01％ to 1％.The dominant reactive oxygen species is OH,whose peak density is about two orders of magnitude higher than that of O.Besides,the densities of OH and O radicals increase with the increasing WVC.It is shown that the formation mechanism of O radicals is significantly affected by the WVC.The dominant reaction creating O radicals changes from the charge exchange between He2+ and H2O to the electron impact dissociation of H2O as the WVC increases from 0.01％ to 1％.This study is helpful for better understanding the application of non-thermal plasmas discharges in water,such as biomedical,environmental engineering.