A method of responsibility proration among multiple three-phase unbalanced sources based on the reference impedance method is proposed． A mathematical model of responsibility proration among multiple three-phase unbalanced sources is developed， four quantitative indices are defined， and the detailed implementation steps are given． The validity and rationality of the method are proved by a simulation system model with three users and the measured data of a 10 kV power system in a steel plant． Finally， the sensitivity analysis between reference impedance and quantitative indices shows that the error of the method is small owing to the improved accuracy of the responsibility proration among multiple three-phase unbalanced sources.