In small hydroelectric power system, the main supply line fault of connected system leads to switch trip on both sides, then the small hydroelectric power system forms isolated network operation and it will appear high frequency and high pressure, which will damage electrical equipment of user, at the same time affect the check without pressure auto-reclosing of power systems. According to these phenomena and based on past research of over-voltage protection device and small hydropower status, this paper proposes a suitable splitting device for small hydropower. It is suggesed to install a new type of auto-disconnection safety for the small hydroelectric power system. It monitors the frequency and voltage of own of auto-disconnection safety in exceptional circumstances station and quickly disconnects itself from system in abnormal situations to avoid damage to electrical equipment and improve power supply quality as well as the reliability of the system operation. Experiments prove that the new method can improve the stability of the grid and the reliability of power supply.