Accurate evaluation of the pollution magnitude in a water body is an important premise for effective water pollution prevention and control. This study used Monte Carlo simulation method together with comprehensive water quality index (CWQI) method and statistical analysis of measured water quality parameters to assess water quality. The Wen-Rui Tang River watershed was used as the study site. Through the combined model and water quality data from 14 monitoring sites at Wen-Rui Tang River during 2004 to 2010, the probability of each site for every pollution level and the influence of each water quality parameter on water pollution were quantified. The results of Monte Carlo-CWQI coupled model indicated that the water quality of Wen-Rui Tang River was highly impaired. The probabilities of the water impairments at sites of Qinfen, Jiushan, Dongshuichang, Shizihe, Nanbaixiang, Huiqiao, Xinqiao, Mishaiqiao, Xianmen, Guangming, Guoxi, Quxi, Xi-ao, and Wutian being at the heavy pollution level were 28.50%, 0.55%, 92.71%, 59.73%, 78.85%, 39.38%, 78.87%, 83.09%, 65.32%, 78.08%, 0.00%, 0.96%, 68.09%, and 86.06%, respectively. The probabilities of the water impairments at these monitoring sites being worse than heavy pollution level were 71.28%, 0.01%, 4.33%, 39.76%, 21.07%, 60.59%, 4.42%, 12.41%, 11.02%, 21.24%, 0.00%, 0.02%, 1.42%, and 13.12%, respectively. The spearman rank correlation coefficient for total nitrogen (TN), ammonium-nitrogen (NH₃-N) and dissolved oxygen (DO) respectively ranged from 0.41 to 0.76, 0.25 to 0.63 and 0.14 to 0.66, which were more than twice on the values for other parameters. This result implied that TN, NH₃-N and DO were the dominant factors affecting the rate of reaching water quality standard in Wen-Rui Tang River. This investigation can broaden the viewpoints for researches and managers on river water quality evaluation and can provide abundant information for decision-making on water environment management.