The invention belongs to the technical field of smart grid operation and stability control, and specifically relates to a method for determining the value range of the third harmonic current of a distributed power flow controller to ensure the safe, stable and economical operation of a system including the distributed power flow controller. The third harmonic current is an important parameter forrealizing the functions of the distributed power flow controller. The invention provides mathematical relationships between the third harmonic current and the transmission line transmission capacity,between the third harmonic current and the insulation voltage of a parallel-side transformer neutral point, and between the third harmonic output efficiency of a parallel-side single-phase converter and the insulation voltage of the parallel-side transformer neutral point, and concludes the method for acquiring the value range of the economical and safe third harmonic current, and provides theoretical basis and reference for device parameter design.