Abstract: The important foundation for the efficient operation of carbon trading mechanism is the objective,accurate and credible carbon emissions data.At present,the emission factor method is often used to account the greenhouse gas emissions,which has many disadvantages such as low accuracy,low efficiency of collecting data,and severe timeliness lag.Compared with the emission factor,the direct monitoring has the advantages of measure convenience,real-time and high efficiency of collection,less human intervention and so on.But there are few researches and applications on this aspect in China.Therefore,in this paper,the methods of the direct monitoring for coal-fired power generation enterprises were summarized,and a carbon emission online monitoring system for thermal power enterprises was developed. It was successfully applied in a 320 MW cogeneration enterprise,which could achieve online monitoring of major emission parameters such as total carbon emissions,carbon emission rate,etc.,and the statistical of historical data. Based on the carbon emission data monitored by the system,the results of direct monitoring and accounting of carbon emissions were compared,and the reasons for the differences of each other in this paper were analyzed.Meanwhile,with many production datas from the enterprise such as coal consumption,the low-level heat generation of coal,the power supply,the heat supply and so on,the relationship among the direct monitoring of carbon emissions and boiler output energy,carbon emissions per unit of product and boiler efficiency was calculated and analyzed.The results show that the carbon emissions of direct monitoring are about 5%-30% less than accounting of carbon emissions,which is 500-1 500 tons less per day.In addition,there is a positive correlation between direct monitoring of carbon emissions and boiler output energy,so their changing trends are basically identical,it can also reflect the changes of boiler operating conditions. There is a negative correlation between carbon emissions per unit product and boiler efficiency.On-line monitoring system for CO_2 reported in this manuscript has a good application prospect in fixed source emission enterprises such as cement,ceramics,steel and petrochemical industries and so on. It can significantly improve the enterprise' s level of the carbon emission data management and low-carbon production assessment.Moreover,it can provide a strong support of data and technology for greenhouse gas emission reduction and the operation of carbon emission trading.