Carbon emission is the final standard to test green building, and it's highly correlated with PM2.5. So, it's very important to establish a clear urban carbon reduction target to research accumulation and assessment framework of carbon emissions data, also, it's more important to establish a calculation model with high reliability of carbon emission used to evaluate the data. The carbon emission of construction is an important criterion to evaluate low-carbon index of construction. In this article, it takes a detailed analysis of carbon emission factors in. the construction life cycle with construction phase, usage and maintenance phase, and demolition recovery phase. At the same time, by the contrast and analysis, we can see that carbon emissions of renovation projects are much smaller than that of new projects, and making recommendations for reducing emissions.