[Objective] The sampling data from 24 cross sections of 5 monitoring sites, including Sanshenggong, Sanhuhekou, Baotoudengkou, Zhangliwenyao, Lama Bay, were used for studying the water quality of Inner Mongolia section of the Yellow River.[Methods] Choosing the dissolved oxygen(DO), chemical oxygen demand(COD), ammonia nitrogen, total phosphorous(TP) and total nitrogen(TN) to evaluate the water quality of the Yellow River by means of comprehensive assessment of fuzzy mathematics.[Results] DO was between 7.2 and 8.1 mg/L, which indicated that the water quality of the Yellow River belong to class I. COD varies between 17 and 33 mg/L, indicating the water quality between the class Ⅲ and Ⅴ. Ammonia nitrogen varies between 0.5 to 1.3 mg/L, water quality was within the classⅡand Ⅲ. TP was between 0.12 to 0.28 mg/L, water quality between the class Ⅱ and Ⅲ. TN was between 1.4 to 2.7 mg/L, and water quality was the class Ⅳ, even occasionally worse than that of the classⅤwater quality standard. COD and TN were the main pollutants. The water quality of Yellow River in Inner Mongolia are roughly in the class Ⅲ or Ⅳ, and its vale varies between 3.3 to 4.2 mg/L.[Conclusions] Through the evaluation, it is found that the water quality in the Inner Mongolia section of the Yellow River is gradually deteriorating. The main reason is the import of foreign pollutants.