The invention provides a jet enthalpy increasing system which comprises a compressor (1), a condenser (2) connected with a compressor outlet (11), and a flash evaporator (3) connected with a condenser outlet (21), wherein the flash evaporator (3) also comprises a first outlet (31) which is connected to a secondary inlet (12) of the compressor; and at least one first expansion valve (4) is arranged between the secondary inlet (12) and the first outlet (31) of the flash evaporator. The jet enthalpy increasing system can effectively prevent the refrigerant from backflow, enhances the stability of the air conditioning unit and especially the heat pump system, can not influence the normal stable operation of the air conditioning unit and especially the heat pump system, can effectively enhance the enthalpy value increment on the basis of the original system, and effectively increases the sum of the system internal energy and external volume work application capacity. The invention also provides an air conditioning unit with the system and an enthalpy increasing control method.