The invention discloses a facultative anaerobic Rhodopseudomonas strain which is collected in China Center for Type Culture Collection, and the collection number is CCTCC NO:M2012366. The Rhodopseudomonas strain can inhibit Pseudoperonospora cubensis Rostov and promote cucumber growth. When in use, a photosynthetic bacteria liquid can be directly sprayed, or prepared into a fermentation liquid and sprayed. The preparation method of the photosynthetic bacteria liquid and photosynthetic bacteria fermentation liquid comprises the following steps: activating the Rhodopseudomonas strain; inoculating the activated and cultured single colony into a serum bottle, and carrying out seed culture; carrying out production culture to obtain the photosynthetic bacteria liquid; and centrifugating the photosynthetic bacteria liquid, collecting the supernate, and filtering through a biofilter to obtain the photosynthetic bacteria fermentation liquid. The method has the advantages of low production cost, favorable antagonistic effect, high efficiency, no toxicity, environmental protection and the like.