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.