A suit of junction box prototype system which is based on underwater junction technology was designed to implement long-term continuous observation for deep seafloor observatory network. Based on analysis of the junction box framework, a waterproof pressure-resistant multi-housing was chosen as mechanical and electronic integration encapsulation structure of the underwater junction box, and the mechanical and thermal structures were designed for the pressure-resistant housings. On the basis of experiments, a new method by arraying MOSFET parts in series on the input side and in parallel on the output side was proposed to transform 2 kV high-voltage DC to 400 V middle-voltage DC, consequently the deep-sea long-distance high-voltage DC power transmission was implemented successfully. Considering the real-time request of the deep-sea observation equipments, the optic Ethernet technology based on TCP/IP and the application layer user-defined protocol technology were chosen to implement communication control. The results of pressure experiments showed that the junction box prototype system can work normally at 40 MPa. As results, the junction box prototype system is proved to be reasonable and practical, which is expected to be used for the implementation of long-term continuous observation for deep seafloor observatory network．