Power system is the main source of carbon emission in China. Studying the carbon emission theory of power system and improving the accuracy of analysis results is an important premise to improve the construction of carbon trading market and promote power carbon emission reduction. In this paper, a calculation method of power system carbon emission flow based on power flow distribution matrix is proposed, which aims to solve the deficiency that the existing methods are too extensive and cannot accurately allocate power and carbon flow. The main idea is to combine the accurate power flow calculation results, adopt the principle of proportional distribution, and construct the power flow distribution matrix by analyzing the flow process of power in the network, so as to accurately establish the relationship between unit power and node flow power. Then, according to the different types of generator units, considering the influence of technical level and operation status of the unit, an accurate carbon emission model of the thermal unit is constructed. Based on this model, using the characteristics of carbon emission flow attached to power flow distribution, the unit carbon emission is allocated to each node load, branch power and network loss, and the accurate traceability and tracking of carbon emission flow in power system are successfully realized. 4-node and 30-node systems are used to test, and the test results verify the correctness and effectiveness of the proposed method.