Abstract: At present, the open of monitoring data is the important working of monitoring system.Significant digits indicate the reliability of date.Dissolved oxygen (DO) is a routine environmental monitoring item.In this paper, we discuss and calculate the significant digits of dissolved oxygen in seawater by iodimetry.The results show that the concentration of Na₂S₂O₃ and the volume of Na₂S₂O₃ and water sample are the three key influence factors.And in general, the concentration of Na₂S₂O₃ is three digits; and the volume of Na₂S₂O₃ is three or four digits; the volume water sample is five digits; the DO measure results are three digits.