A logging device and method for locating a watered zone in a horizontal well with high watercut problem by isotope labeling is provided. The method successively includes the following operations: a. solidifying an isotope used in an isotope releasing device into a water-soluble carrier; b. connecting the isotope releasing device with a tubing and setting them to a horizontal segment of the well; c. dissolving the water-soluble carriers in the water at the bottom of the well to release the isotope, wherein the releasing rate is proportional to the flow rate of the water; d. starting a lift-up pump and beginning a production logging; and e. continuously performing γ-ray energy-spectrum analysis on produced water on the ground, so as to locate the watered zone of the horizontal well by the detected species and characteristic content of the isotope, thereby reduces operation risk, saves platform occupying time and logging cost.