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An environmental and economical assessment model is developed, in order to evaluate the performances of transcritical CO2 heat pump system with dedicated mechanical subcooling (CO2 HPDMS). Introducing DMS to traditional CO2 HP system is an efficient method to reduce the primary energy consumption, which can be further decreased by using small temperature difference fan-coil unit (STD-FCU) as heating terminal. Using CO2 heat pump system for space heating is an environmentally-friendly heating method. The corresponding pollution emissions are only inferior to those of the wall hanging gas heater. The initial capital cost and operating cost of CO2 HPDMS system are both lower than those of CO2 HPBASE system, and the CO2 compressor cost accounts for about 80% of the overall initial capital cost. In contrast to other traditional heating methods, the payback periods of CO2 HPDMS system are not more than 9 years in most cases. If the CO2 compressor and electricity price are reduced by 20% and 28.79% respectively, the life cycle cost of CO2 HPDMS will be competitive to that of coal-fired boiler. In China, it is a promising way to adopt CO2 HPDMS for space heating in the near future with the assistant of electricity price subsidy and compressor price reduction.