This paper aims to investigate the factors that can mitigate carbon-dioxide (CO2) intensity and further assess CMRBS in China based on a household scale via decomposition analysis. Here we show that: Three types of housing economic indicators and the final emission factor significantly contributed to the decrease in CO2 intensity in the residential building sector. In addition, the CMRBS from 2001-2016 was 1816.99 MtCO2, and the average mitigation intensity during this period was 266.12 kgCO2/household/year. Furthermore, the energy-conservation and emission-mitigation strategy caused CMRBS to effectively increase and is the key to promoting a more significant emission mitigation in the future. Overall, this paper covers the CMRBS assessment gap in China, and the proposed assessment model can be regarded as a reference for other countries and cities for measuring the retrospective CO2 mitigation effect in residential buildings.