Abstract: Under unbalanced working conditions, existing dynamic phasor theories are difficult to support efficient modeling of the type of power electronic equipment controlled in the dq rotating coordinate system. To address the above problem, the dq-sequence dynamic phasor (dq-SDP) method is proposed in this paper. First, by performing the instantaneous symmetric component decomposition on a set of three-phase time-domain signals and introducing Fourier decomposition based on the reference angle of Park transform, the definition of the dq-SDP is given. Then, the multiplication characteristics of the dq-SDPs are derived. Also, the unified form, derivation steps, and fast formation method for the dq-SDP state equations are provided. Finally, the effectiveness of the dq-SDP modeling method is verified by modeling the one-terminal voltage source converter under unbalanced operating conditions. The work in this article supplements the existing dynamic phasor theory, providing a new method for efficient modeling of a class of power electronic devices under unbalanced operating conditions.