Cabled undersea observation networks have been attracting increasing research attention due to the abilities of abundant power distribution and broad bandwidth communication. Stability of their power supply systems is crucial for their normal operation. However, rare studies were carried out on constant current （CC） mesh-like undersea observation networks. This paper focuses on designing branching unit (BU) for CC mesh-like seafloor observation network. Firstly, the design concept of CC mesh-like submarine power system is introduced, as well as the equivalent model of power delivery submarine cable and the circuit of CC converters. Then, based on features of the power network, concept of BU for mesh-like CC system is proposed respectively, and reliability protection schemes for various scenarios are analyzed. The model of the observatory network was set in MATLAB, Simulations were carried out, from which results showed good consistence, verifying the theoretical methods.