Abstract: During the equipment installation in the Nuclear Power Plant reactor containment, it is not easy to measure the matched components' size for the narrow installation space but with high precision. Previously, the Red Lead Impression Indirect Measurement was used, but it is not suitable for high-precision measurements; and there used to do the measurement directly by personnel in smaller shape, but it is of high risk and hard to realize. The paper here proposes a non-contact narrow gap measurement technology which systematically discusses the measurement scenario, error analysis and measurement requirements, and proposes a special measurement system and measurement method suitable for the measurement scenario. The feasibility of this technology is verified by simulation test and measurement of actual product. Through this technology, the remote high-precision measurement of non-reachable parts is realized, and the measurement accessibility and safety of precision assembly are improved.