Abstract: In order to meet signal transmission requirements of the thermal neutron detector in nuclear instrument system (NIS) of nuclear power plants, a tetrad coaxial organic cable assembly was developed to use inside the containment, and a design scheme for the cable and matched connectors was presented. According to characteristics of transmission signals and special usage environment inside the containment, NIS functional requirements, as well as regulations for nuclear safety cables, tests of this cable assembly were divided into three categories, which were cable material performance tests, electrical performance tests, and special tests. Detailed test items, inspection requirements, and test results were introduced, which could give advises for the development of similar products.