The invention provides a frozen earth boundary control system which comprises a low temperature saline water circulation subsystem, a hot saline water circulation subsystem and a temperature monitoring subsystem. The low temperature saline water circulation subsystem comprises a cooling water device, an ammonia circulation device and a low temperature saline water device which are connected in sequence. The ammonia circulation device comprises an evaporator, an ammonia compression engine, a condenser and a throttle valve which are connected in sequence to form a loop tank. Heat exchange is carried out between the ammonia circulation device and the cooling water device through the condenser, and the heat exchange is also carried out between the ammonia circulation device and the low temperature saline water device through the evaporator. The low temperature saline water device comprises a freezer which is used for being placed in a soil body outside the well wall. The hot saline water circulation subsystem comprises a hot saline water circulation device which is used for being placed in a soil body between the freezer and the well wall. The invention further provides a method for controlling the frozen earth boundary. By means of the frozen earth boundary control system, the advance speed of a frozen frontal surface towards the direction of the well wall can be controlled, the well wall is enabled not to be affected by large frost heaving pressure, and the safety of the well wall is protected.