This paper focuses on the effect of introducing clean energy in compact city. As is well known, carbon-dioxide emissions from burning gas are about half the level from coal. It is cleaner to generate electricity with natural gas than coal. When it is used for combined heat and power (CHP) system, utilization ratio and utilize benefit could be advanced considerably. This paper chooses a case in Changchun to research the effect. Three energy supply systems are set up, namely boiler system (system A) and two combined heat and power systems (system B and system C). The intensity of energy consumption of Changchun could be reckoned according to the intensity of energy consumption of Tokyo and the ratio of Degree-day of the two cities. Likewise, equipment efficiency, equipment price, energy price, CO2 emission intensity and depreciation rate are postulated. According to calculated and given data to calculate primary energy consumption, CO2emission, initial cost, annual operation costs and payback periods. The results are as follows: CHP systems (system B and system C) energy saving rates are respectively 22.9% and 8.0%, CO2 reduction rates are respectively 24.6% and 10.0%, payback periods are respectively 7.8 and 4.3 years relative to the boiler system (system A). Comparing the results of three systems, it could conclude that CHP systems (system B and system C) using natural gas would be attractive options when introducing energy system in compact cities.