Abstract: For the problem of open-circuit fault with the optoelectric cable in the seafloor observatory network, a method of its identification by measuring the mean residual value is proposed. According to real-time network topology and current direction, an adjacency matrix of directed graph is built, to divide the network structure into three different areas. The voltage difference of the junction box across the moment the fault occurs is calculated and used for open-circuit fault section location. A hypothesis-testing method for the coupled regions is proposed to locate the section of the open-circuit fault. A model for the seafloor observatory network is developed and the open-circuit fault is analyzed by simulation. The results show that the method proposed is practical and efficient.