Abstract: In this paper, the research and development of water gap detection system of plate type spent fuel assembly in hot chamber based on eddy current method was carried out, the water gap detection process of spent fuel assembly was determined, and the mechanical components such as special water gap measurement probe, three-dimensional motion device, visual alignment mechanism, four-axis motion mechanism and assembly positioning device were developed to realize the accurate measurement of channel gap of plate type spent fuel assembly. Through the hot chamber measurement verification of spent fuel assembly of CARR (China Advanced Research Reactor), the detection range of this equipment is 0.9-2.9 mm, and the detection accuracy was ±0. 02 mm. The measurement of hot chamber channel clearance is realized for the first time, and the measurement accuracy is consistent with that of conventional components.