Abstract: The intelligent internal inspection show that many corrosion defects were found in the front and middle sections of an oil, gas and water mixed transportation uninsulated subsea pipeline. The corrosion causes of the subsea pipeline were analyzed by simulated corrosion experiment, pigging products and microbial test analysis, and temperature change curve analysis. The results show that a lot of number of corrosion defects in the front and middle sections of the pipeline were caused by the interaction between SRB and TGB which induced corrosion and underdeposit corrosion. The different locations of the defects in the subsea pipeline at different time were caused by temperature drop along with pipeline because of the change in the inlet temperature. In recent years, measures such as adding pigging, adjusting the injection of bactericide and monitoring the effect of bactericide have been taken, the corrosion of subsea pipeline has been alleviated, and the corrosion has not accelerated significantly.