Along with the economic development, more and more asymmetric loads, such as electric railway, are connected to the power grid. It is very important to analyze the pollution of these negative-sequence sources which will deteriorate power quality seriously and to design the method to improve power quality. Therefore, how to establish the negative-sequence source model is meaningful. A new method named Combined Negative-sequence Source Modeling is presented. The Negative-Sequence Source model is aimed to the load-bus of power system. The negative-sequence pollution in the whole power grid could be analyzed by this model. A negative-sequence source model based on the measured data and an practical power system example is given to analyze the asymmetric power flow with PSASP. The results proved the correctness and engineering practicality of the modeling method.