The “bucket type” installation of ex-core neutron detector has been widely adopted in advanced third-generation nuclear power plants. This installation method hoists the detector assembly into the guide tube embedded in the periphery of reactor core by wire rope, and gradually descends to designated location under the gravity. Since the detector location inside the guide tube is invisible, it could only be preliminary evaluated by the length of wire rope. However, the guide tube possesses multi-curved structures, in which the state of the wire rope is unknown, and meanwhile the wire rope will be stretched under the gravity of the detector assembly, which also leads to deviation between the location evaluated according to the length of wire rope and the actual detector assembly location. The deviation has an influence on the measurement accuracy of the ex-core nuclear measurement system, which could pose a failure risk to the safety function of the nuclear power plant. This paper provides a method for accurately measuring the location of detector inside the guide tube, and the measured positioning accuracy could be controlled within ±50 mm for source range and intermediate range detector, and the accuracy for power range detector is controlled within ±10 mm. This method has already been completed in a third generation nuclear power plant in China. It not only improves the reliability of ex-core nuclear measurement system, but also reduces the locating risk introduced by detector assembly in the case of replacement and maintenance. At the same time, the method can save construction manpower, reduce installation cost and provide good practice for similar projects.