Abstract: There is complex energy interaction among substations, powering trains and braking trains in urban rail transit system. The existing power flow calculation methods only focus on obtaining the current of each branch at a certain time, while neglect the quantitative current and power distribution relationship between sources (substation, braking train) and loads (powering train, line resistance) in the system. This paper proposes upstream and downstream current tracing of urban rail transit based on circuit theory to quantitatively characterize the proportional relationship between source current and load current. Based on the result of current tracing, upstream and downstream power tracing are carried out and the distribution coefficients between the power of sources and loads can be obtained. Finally, a case study is carried out to verify the proposed power flow tracing method. Through power flow tracing, the power flow path of the system can be displayed visually and quantitatively, providing a theoretical support for the optimizing sizing and coordinated control of multiple energy storage systems.