In order to study a mass culture method for photosynthetic bacteria Rhodopseudomonas, from January 5, 2015 to April 26, 2016, the freshwater compound fish feed with tap water was put in plexiglass columns in a greenhouse for fermentation, and neither other substances nor bacteria strain was added during the culture. During the experiment, with the decay of the feed, a kind of dark red substance gradually appeared on the inner wall of the container, and the attachment area gradually spread to the entire inner wall, and then the water body gradually turned to dark red. The results showed that the highest content of TN, TP and TOC in the water was nearly 1 200 mg/L, 700 mg/L and 2 700 mg/L, respectively, during the feed decay, and then they decreased. The high throughput sequencing analysis for the microorganisms showed Rhodopseudomonas was the most dominant genus in either the attached microorganisms or the planktonic microorganisms, and the relative abundance reached 73.19% and 54.31%, respectively, when the water was dark red. It showed that the high density culture of the attached or planktonic Rhodopseudomonas was achieved. The massive culture mechanism for Rhodopseudomonas in this experiment was deduced here:firstly, the compound fish feed was degraded into micromolecule substances with heterotrophic bacteria, followed by the concentration of nitrogen, phosphorus, carbon, metal ions, vitamins, trace elements and other micromolecule, which met the demand of the high density growth of Rhodopseudomonas with the conditions of appropriate water temperature, light and other physical factors, and then Rhodopseudomonas massively grew to become the dominant genus. This research provides a method for mass culture of Rhodopseudomonas with fermentation of aquatic compound feed and water, which also provides useful information for the large-scale culture of other heterotrophic bacteria that can biodegrade macromolecules.