Abstract: In accordance with China’s “three-step” strategy for nuclear energy development, establishing mature commercial fast reactors and fast reactor spent fuel reprocessing technology is a must for the “second step” of China’s nuclear energy development. The integrated fast reactor nuclear energy system is regarded as a realistic choice for the development of advanced closed fuel cycles, and the key technology of molten salt electrolysis dry reprocessing is an important part of it. The molten salt electrolysis dry reprocessing technology is suitable for processing high-burnup metal and MOX spent fuels from fast reactors, avoiding problems such as the easy radiolysis of anhydrous solvents and the generation of a large amount of radioactive waste liquid. At the same time, it has the characteristics of a wide range of processing objects, short process flow, compact equipment, and low criticality risk. The operating temperature of the molten salt electrolysis dry reprocessing process is about 450-500 ℃. The high-temperature chloride molten salts will corrode the process equipment and pipeline materials. In addition, after the high-temperature molten salts cool down, they will adhere to the surface of the process equipment and pipelines. The above two situations will lead to the deposition of radioactive materials in the molten salts in the process equipment and pipelines. Local deposition of a large amount of radioactive materials in process equipment and pipelines will affect the safety of personnel and process operation. Therefore, it is necessary to carry out online monitoring of the deposition of radioactive materials in key process equipment and pipelines of molten salt electrolysis dry reprocessing. The molten salt electrolysis dry reprocessing equipment is compact, and its process equipment and pipelines are mainly located in sealed boxes such as glove boxes and hot cells, where the space is relatively small. Conventional neutron REM meters are relatively large in volume and are not suitable for installation in glove boxes and hot cells with densely distributed process equipment and pipelines for online monitoring of deposition. Therefore, based on the information of the neutron spectrum in molten salt electrolysis dry reprocessing, the structure of the sealed box, and the parameters of the process equipment, this paper developed a small neutron detection system for dry reprocessing pipeline deposition source items, and tested its measurement performance (calibration factor, relative inherent error, repeatability), dust-proof and water-proof grade, and electromagnetic compatibility performance. The test results show that its calibration factor for dose rate measurement of ²⁵²Cf neutron source is 1.07, the relative inherent error for dose rate measurement of ²⁵²Cf neutron source is −6.5%, and the repeatability of measurement data is 0.1%. The detector’s dust-proof and water-proof grade is IP68, and it passes the electromagnetic compatibility test specified by the nuclear instrument electromagnetic environment conditions and test methods. This system can provide technical support for online measurement of the deposition of radioactive materials in dry reprocessing process equipment and pipelines, so as to ensure the safety of personnel and process operation.