The invention provides a constant-temperature cold water unit and a control method thereof. The constant-temperature cold water unit comprises a water tank, a refrigerating unit, a controller, a water tank temperature sensor and a water tank temperature controller, wherein the refrigerating unit is a closed water tank refrigerating loop consisting of a compressor, a condenser, an expansion vale and an
evaporator sequentially; the compressor, a second
solenoid valve and a gas-liquid separator are sequentially composed to be a closed idle loop; the compressor, the condenser, a third
solenoid valve, a capillary tube and the gas-liquid separator are sequentially composed to be a closed spray liquid cooling circuit. The constant-temperature cold water unit further comprises a heater; and the water tank temperature controller comprises two PID (
Proportion Integration Differentiation) temperature controllers. According to the invention, when the temperature of the water tank is constant, the compressor and the gas-liquid separator are configured to be the idle loop, so that the
energy consumption is saved; when the temperature of the compressor exceeds a set value, the spray liquid cooling circuit can be opened for cooling the compressor. In the constant-temperature cold water unit, two PID temperature controllers are adopted, therefore, the
water temperature is ensured to be constant, and the
water temperature stability of plus or minus 0.1 DEG C can be reliably ensured.