Raw water in the Nanchang district of Ganjiang River and water after conventional treatment were estimated by the method of the ultra filtration to find out its molecular weight distribution. According to the result, the molecular weight distribution of the raw water in Ganjiang River shows the trend which has the large edges and small middle part, that is to say; the organic matter mainly centralizes in a scope whose molecular weight are larger than 30ku and less than 1ku, the middle part occupies a relatively small proportion. In winter low-water season, there is little organic matter in the river and the small molecular of the organic matter plays a dominant role. In the season of high-water, the amount of organic matter increases, and while the small molecular of organic matter decreases and large molecular of organic matter becomes a large part. The popular water purifying process in Nan Chang city is just to mainly remove the large molecular in the organic matter, which leads to a bad result，the low efficiency of removing the small molecular.