The work aims to study origin of H2S and tubing corrosion mechanism during offshore oilfield waterflooding, which is of great importance in effectively preventing generation of H2S and selecting anti-corrosion materials of tubing. Firstly, chemical detection was applied to gas samples, injection water samples, oil samples and wellhead corrosion inhibitor. Secondly, sulfate-reducing bacteria (SRB) culturing verification test was done, SRB growth characteristic was study, and finally corrosion behavior of discarded L80 tubing was analyzed. There was some H2S in the gas sample, and H2S concentration in some production wells was up to 0.03%. However, no sulfide was detected in the water sample, aqueous phase constituent of injection water and oil samples as well as the wellhead corrosion inhibitor, possibility of carrying H2S in the injection process was excluded. SRB colonies found in the PGC media showed that SRB was contained in water produced from stratum. The temperature of 55~65 ℃ and pH of 5.5~6.0 were optimum to grow SRB colonies in this oilfield. H2S concentration had been very low (<0.5 mg/L) after NaNO2 was added and the SRB concentration began to increase after 192 h, showing that the inhibitory effect of NaNO2 was very good. Crack width of discarded L80 tubing was as narrow as 20~50 μm, the narrower the crack was, that the lower the risk of sulfide stress cracking (SSC) would be. The depth of corrosion pit was less than 50 μm, no large and deep pit hole was detected on the surface, indicating that the corrosion rate of L80 tubing was low. The SRB can reduce SO42? to H2S by [H] generated in biofilms in anaerobic conditions, so the H2S is secondary during waterflooding, generation of H2S in the oil field is primarily caused by unqualified injection water. L80 tubing is subject to microbial corrosion (MIC) in SRB-CO2 corrosion system, and the corrosion of secondary H2S on the tubing is less severe than that of primary H2S. It is recommended to select oilfield anti-corrosion material for tubing based upon secondary H2S in later oilfield development, lower level of anti-corrosion and save operation cost. The study result is of good promotional value.