In this study, we investigate the effect of residual H2O pressure (P-H2O) on the structural, electrical, and optical properties of indium tin oxide (ITO) film and the current-voltage (I-V) performance of silicon heterojunction (SHJ) solar cells. The ITO films and SHJ solar cell were prepared under different P-H2O (3.75 x 10(-7) Pa, 6 x 10(-7) Pa, 10.25 x 10(-7) Pa) by varying the pumping time before starting the fabrication process. The experimental results revealed the crystallinity of ITO films was suppressed with the increase in P-H2O. Further, the sheet resistance of ITO films was reduced from 103 O/sq to 60 O/sq due to the enhanced carrier mobility, and the transmittance in near-infrared (NIR) region exhibited a slight increase, but the conversion efficiency of SHJ solar cells dropped dramatically from 23.3% to 22.79% due to the degradation of fill factor (FF). The analysis on n-a-Si:H/ITO and ITO/Ag contact properties indicated that such FF degradation could be attributed to the increased specific contact resistance for the two contacts, when P-H2O was high. Finally, SHJ solar cell with conversion efficiency of 23.53% was achieved by restraining the residual H2O vapor during ITO deposition along with the optimization of transmittance in NIR region by adjusting the amount of O-2 flow.