Abstract: A comprehensive set of methods was established based on Mann-Kendall test and rescaled range (R/S) analysis for analysis of variation trends of water quality in the past and future. Firstly, seasonal Mann-Kendall tests were performed to determine variation trend of water quality time series in the past. And then, R/S analysis was done used to calculate Hurst indexes (I_H)of the water quality time series(0< I_H< 0.5, 0.5< I_H< 1.0 and I_H=0.5 indicates anti-persistence, persistence and white noise characteristics, respectively). And in the end, variation trend of water quality in future was extrapolated by taking into comprehensive account the water environment protection program and its progress. Furthermore, this approach was demonstrated using the data of TN concentrations and COD_Mn in the Shanxi Reservoir and its six tributaries during the period of 2010-2014 as an example. Results show that water quality in most of its tributaries will stop deteriorating or go on improving in future, but TN in the reservoir will not display any declining trend, because the reservoir has a high TN retention capacity and numerous pollution sources. The reservoir has a long way to go before its water quality is fundamentally improved. In addition to keeping on controlling incoming pollution from its tributaries, countermeasures should be taken to control pollution of the water-level-fluctuating-zone and its endogenous pollution.