Based on 2005-15 water quality data and measured flood data in 2006-12 at Tongguan Bridge Section, storm-period water-quality characteristics and different time-scale water-quality characteristics have been analyzed for the Weihe River and its basins. The interannual and quarterly mean concentrations of COD, NH₃-N, DO, and river pH value are calculated to evaluate pollution characteristics. Water quality in flood and non-flood periods were monitored to explore the relationship between peak flows and water quality in stormwater. The results showed that in the stormwater period, index concentrations and their fluctuation ranges decreased and were reduced gradually with the increase of peak flow. When peak flow was in the range of 480-680m³/s, concentrations of COD and NH₃-N had the same trend of increasing and decreasing in general. During 2005-15, river pH value fluctuated in the range of 7 to 8, and pH value in the second quarter was slightly higher than in the other three quarters. Water quality of the Weihe improved significantly after 2010.