Establishment of an accurate and efficient converter mathematical model is a key problem in studying the dynamic characteristics of large-scale ac-dc hybrid systems. In this paper, the commutation process of commutator in the case of asymmetrical fault is analyzed, and the zero-crossing migration and on-angle migration of commutative voltage caused by phase shift of commutative voltage are considered synthetically, as well as the influence of harmonics of AC system on the duration of commutation, an improved method for calculating the dynamic phasor of commutation angle is established, and an improved dynamic phasor model suitable for asymmetric operation is proposed. The accuracy of the proposed improved convertor dynamic phasor model are verified by simulation, which can be further extended to the modeling under various fault conditions and has a certain universality.