The invention discloses a cold quantity combination control method and a cold quantity combination control device for a constant-frequency air-conditioning unit. The method comprises the following steps: determining a cold quantity combination value set of the constant-frequency air-conditioning unit; detecting the current cold quantity combination value of the constant-frequency air-conditioning unit; detecting the temperature or return air of the constant-frequency air-conditioning unit; judging whether the temperature of the return air is greater than a first preset threshold value; if the temperature of the return air is greater than the first preset threshold value, adjusting the cold quantity combination value of the constant-frequency air-conditioning unit from the current cold quantity combination value to the cold quantity combination value, which is closest to the current cold quantity combination value, of a plurality of cold quantity combination values. Through the method and the device, the problem that the service life of a compressor is influenced by starting and stopping the compressor frequently when the constant-frequency air-conditioning unit is subjected to the cold quantity combination control is solved, so that the cold quantity combination value of the constant-frequency air-conditioning unit is adjusted from the current cold quantity combination value to the cold quantity combination value, which is closest to the current cold quantity combination value, of a plurality of cold quantity combination values when the temperature of the return air is greater than the first threshold value; therefore, the effect of prolonging the service life of the unit is achieved.