Frequent corrosion perforation and monitoring effectiveness deviation have been observed in the undulating segments of high-water-cut crude oil gathering pipelines. This study proposes a corrected model and charts for in-pipe probe monitoring effectiveness (defined as the ratio of probe-measured corrosion rate to the circumferential maximum wall corrosion rate at the same cross-section). Computational fluid dynamics (CFD) simulations and loop experiments were conducted to systematically investigate the effects of flow velocity (0.4–1.2 m/s), temperature (20–60 °C), and monitoring time on probe effectiveness. The results show that monitoring effectiveness exhibits a “rise-then-fall” trend with increasing flow velocity, reaching 72.88% at 0.8 m/s and decreasing to 50.10% at 1.2 m/s. Elevated temperatures gradually reduce effectiveness, from 74.63% at 20 °C to 67.00% at 60 °C. With prolonged monitoring time, effectiveness significantly increases, rising from 5.15% at 12 h to 76.48% at 36 h. Based on these findings, a corrected corrosion rate model coupling velocity, temperature, and time was established, and charts illustrating corrected effectiveness under different conditions were generated. Field application indicates that the corrected probe monitoring effectiveness reaches 97.36% and 93.83% in the upward-inclined and downward-inclined segments, respectively, with an average improvement of 35.72%. The results provide effective technical support for corrosion monitoring and integrity management in complex undulating pipelines.