A strain of sulfate reducing bacteria (SRB) was isolated from waste water in Changqing gas filed. The cell of strain was found to be Gram-negative, non-spore-forming, rough, and motile by a flagellum. The cell was rod or arc with 0.6—0.8 μm wide and 1.0—5.0 μm long. The strain showed positive reaction in the SRB test bottle. According to the modality and physiology, it was concluded that the strain belonged to Desulfovibrio sp. The quantity of SRB was 9.5×108 per milliliter after the strain was cultured in liquid for two days without Fe2+. The strain reached logarithmic phase after inoculation for 6 hrs, and reached a peak after 48 hrs, then stabilized about 4 hrs, finally entered a period of decline. The corrosion behaviors of 20R steel in SRB solution were studied by electrochemical impedance spectrometry and polarization curve. The results showed that the corrosion rate increased with increasing initial concentration of SRB, but the relationship was not linear. The corrosion rate of 20R steel soaked in SRB solution, 10 times diluent, 100 times diluent and contrast medium for three days was 0.0922, 0.0343, 0.0284 and 0.0281 mm/a, respectively. The corrosion rate of 20R steel soaked in 100 times diluted SRB solution for seven days was 0.0578 mm/a. The corrosion rate increased with increasing immersion time. The promoting role of SRB on cathode was greater than that on anode.