The traditional carbon emission calculation in power industry treats power source as emission source to the neglect of the power transmission process and making it impossible for consumers to directly perceive their carbon emission information through electricity consumption.It is thus necessary to measure carbon emission based on demand level to encourage green power access，calculate consumers' carbon emission and benefit the promotion of green power certificates market.A carbon emission flow tracing model capable of reflecting the indirect effect of reactive power on active power via joint tracing is proposed.The correctness of the proposed model is verified by an IEEE 57-bus system.Nodal emission cuts proportion after introduction of green power can be measured through carbon flow tracing while providing a basis for the allocation of responsibilities for emission reductions between regions.And the carbon emission generated by power losses should be a new reference to low-carbon power dispatch.