The utility model relates to a many first medium heat cascade utilization's double -pipe heat exchanger can be applied to and use the heat pump water heater system of CO2 as the refrigerant, and the high -pressure side heating is used as the system. This double -pipe heat exchanger comprises upper and lower two -layer double -pipe heat exchanger, and upper double -pipe heat exchanger and the double -pipe heat exchanger of lower floor constitute by inner tube and outer tube suit, and upper double -pipe heat exchanger's the inner tube intercommunication double -pipe heat exchanger's of lower floor inner tube is independent enclosed construction respectively between upper double -pipe heat exchanger's outer tube and the inner tube and between the double -pipe heat exchanger's of lower floor outer tube and the inner tube. This double -pipe heat exchanger strides high -quality thermal while of critical CO2 heat pump water heater high -pressure side at make full use of, further absorbs high -pressure side low temperature heat, realizes thermal cascade utilization, and then improves whole energy efficiency ratio of striding critical CO2 heat pump water heater system, realizes energy -conserving effect.