Abstract: An on-line testing method of specimen wall thinning rate was researched and improved based on the previous testing method. A system was built to carry out the flow accelerated corrosion test by this method in simulated secondary water chemistry of PWR. The experimental results show that:the improvement of on-line testing method was reasonable for testing flow accelerated corrosion rate of carbon steel, and it could test continually without changing specimens and reduce the influence of its surroundings; temperature fluctuation had a great influence on the on-line measurement, and non-dimensional voltage could be brought in to reduce the influence.