The invention relates to an air conditioning unit device, in particular to a water source heat pump unit system for a low temperature water source. The system comprises a compressor, an oil separator, a condenser, a oil return injecting device and an evaporator; the evaporator is in a falling film type structure, a liquid distributor is arranged in the upper portion of the evaporator, the top of the evaporator is provided with a liquid incoming pipe, the port of the liquid incoming pipe contacts with the surface of the liquid distributor, the top of the evaporator is provided with an exhaust pipe, the bottom of the evaporator is provided with a discharge pipe, and heat exchange pipe beams are arranged below the liquid distributor. According to the system, water is distributed to the surface of the heat exchange pipes evenly through the liquid distributor, the heat exchange effect is fine, the refrigerant can be evaporated fully, the refrigerant storage amount is reduced, and the heat exchange pipes are prevented from freezing; meanwhile, since the level of the evaporator is low, the mixing ratio of lubrication oil is decreased, the lubrication oil can be injected to the compressor through the oil return injecting device, and the fine oil returning effect is achieved.