The invention relates to a refrigerating and freezing device and a control method thereof. The refrigerating and freezing device comprises a box body, a refrigerating evaporator, a dehumidifying evaporator and an isolating device, wherein a storage room and an evaporator room which are isolated by an air duct cover plate are limited in the box body; a refrigerating air return opening and a dehumidifying air return opening are formed in the air duct cover plate; the dehumidifying evaporator is positioned below the refrigerating evaporator; the isolating device is arranged in the evaporator room and configured in a manner of being controlled to switch among three states: namely a semi-isolating state, a full-isolating state and an opening state; in the semi-isolating state, the air flow flowing through the dehumidifying evaporator is allowed to directly flow into the storage room without flowing through the refrigerating evaporator; in the full-isolating state, the air flow in the space where the dehumidifying evaporator is located is not allowed to flow out; and in the opening state, the air flow flowing through the dehumidifying evaporator is allowed to flow into the storage room after flowing through the refrigerating evaporator. The control method comprises the following steps: dehumidifying, defrosting and refrigerating and refrigerating. According to the refrigerating and freezing device and the control method thereof, moisture in the air flow flowing through the refrigerating evaporator is greatly reduced, the frosting of the refrigerating evaporator is delayed, and the defrosting period of the refrigerating evaporator is prolonged.