A metal-dissolved oxygen seawater battery (SWB) uses metal and dissolved oxygen as the reactants, and it is ideal for use as a long-time low-power distributed power supply in deep sea, due to its advantages of open structure in service without electrolyte. However, several simulating deep-sea environmental factors, such as flow rate, dissolved oxygen concentration, and temperature of seawater may affect the oxygen reduction reaction (ORR) rate and the stability of electrochemically modified polyacrylonitrile-based carbon fiber brush (MPAN-CFB) cathode, which was studied by steady-state polarization and galvanostatic discharge methods. In addition, the scales formed on MPAN-CFB surface were characterized by SEM and XRD. Results show that the ORR rate increased quickly with the increase of the seawater flow rate up to 3 cm/s, and then gradually stabilized. Moreover, the ORR rate was largely affected by dissolved oxygen concentration, and the concentration of >3 mg/L was favorable. Compared with surface layer temperature of 15℃, the low temperature of deep sea (4℃) has a negligible effect on ORR rate. When the working current is too high, it will lead to the formation of CaCO3 scales (aragonite) of at the cathodic surface, resulting in the decrease of ORR rate, and consequently the damage to the long-time stability of MPAN-CFB.