Abstract: To meet the requirements of safety control during the fuel loading and refueling of nuclear reactors, a reactor first loading monitoring system was developed, and the related mechanical structure and electrical structure design and manufacturing of the system were completed including 3He proportional probe and pulse counter device, etc. The probe assembly adopts a new structure. The mechanical structure is a combination of a bridle structure waterproof connector, a waterproof tube and a load-bearing multi-core shielded cable. A fast charge sensitive preamplifier and a high-voltage module were integrated in the probe. The pulse counting device consist of several NIM modular plug-in units is mounted on a NIM rack mounting unit and a cabinet. The system contains four measurement channels and an acousto-optic alarm channel, which can realize functions such as counting rate display and over-threshold acousto-optic alarm. The tests show the system meets the requirements for neutron flux rate monitoring during nuclear reactor fuel loading and has been applied to unit 5 and unit 6 of Hongyanhe Nuclear Power Plant.