To reduce the corrosion failure frequency of oil and gas pipelines, corrosion monitoring coupons are often used to evaluate the internal corrosion status of pipelines. However, the local flow field varies after the installation of the coupons, so the monitoring corrosion rate is different from that of the pipe wall. To evaluate the effectiveness of corrosion coupons monitoring, a lab-scale flow loop system was used to carry out flow corrosion experiments to obtain the local corrosion rates of corrosion monitoring coupons and pipe wall, respectively. Combined with the characterization of corrosion products and multiphase flow, a mechanism model of corrosion monitoring effectiveness was established. It was clear that the deviation of corrosion monitoring rate was related to the flow rate, shear force and turbulent kinetic energy. The accumulation of corrosion media and the detachment of corrosion product films resulted in variation in the morphology and the monitoring effectiveness.