Objective To study the influence of corrosion scales on the corrosion inhibitor efficiency in submarine pipelines, and to give suggestions for the usage of corrosion inhibitors during the operation of submarine pipelines. Methods With a high-temperature and high-pressure reaction vessel simulating the operation condition on site, the corrosion rates of submarine pipeline steel coupons were measured using weight loss method, and the micro-morphology of the test specimen was observed via scanning electron microscopy. The general corrosion rate and inhibition efficiency on both the eroded specimens and the specimens that were eroded but the corrosion scales were erased, were evaluated according to the test results. Results When the test specimen was pre-eroded by CO2 and covered by corrosion-scales, the inhibition efficiency of the high-efficiency corrosion inhibitor H was 75%, which was lower than its inhibition efficiency on the bare steel, i.e. 90%. After corrosion scales were removed, the inhibition efficiency of inhibitor H was 80%, which was higher than the inhibition efficiency on un-cleaned eroded specimen, but still lower than the inhibition efficiency on the bare steel, and also lower than the common design value of inhibition efficiency, i.e. 85%. After corrosion scales were removed, the general corrosion rate of test specimens decreased. Conclusion After submarine pipelines were applied, the CO2 corrosion caused by the incorrect or insufficient usage of corrosion inhibitors should be avoided. If significant CO2 corrosion was detected by testing iron ion content, the measures of pigging and pre-filming should be taken, and the type and amount of corrosion inhibitors should be adjusted.