This paper presents a method on order-reduction of interharmonic analysis based on the principle of interharmonic interaction. First, we obtain the transformation between dynamic phasors in dq coordinate and dynamic phasor sequence components(DPSCs). Then, a full-order interharmonic analysis model is developed taking the voltage and current double closed-loop controller into account via the transformation mentioned before. The principle of interharmonic interaction is studied based on the full-order DPSCs model. On the basis of the principle, the dominant frequencies for all kinds of variables are selected and a reduced-order interharmonic analysis model is derived. Finally, two examples are presented for illustration of the proposed method. It is shown that the reduced-order interharmonic analysis model based on the principle of interharmonic interaction keeps high accuracy with substantial computational savings.