A control method of a transcritical carbon dioxide composite heat pump system is disclosed, wherein the transcritical carbon dioxide composite heat pump system includes: a CO2 main circuit compressor, an air-cooling-air-cooling recombiner, a supercooling-evaporation recombiner, an evaporator and a CO2 auxiliary compressor; wherein the air-cooling-air-cooling recombiner comprises a CO2 main circuit, a CO2 auxiliary circuit and a water circuit; the supercooling-evaporation recombiner comprises a CO2 main circuit supercooling section and a CO2 auxiliary circuit evaporation section. The present invention includes two working modes according to the return water temperature, so that the unit has a wider application range and meets daily needs. There is only one heat exchanger for refrigerant and water. Compared with the three water and refrigerant heat exchangers in the conventional transcritical CO2 composite heat pump, the circulating water circuit is a single circuit with one inlet and one outlet.