The T-type alternate arm multilevel converter (T-AAMC) is one of the most competitive topology for high voltage DC transmission systems, which combines the lower costs and fault ride-through capability. Since the DC arms conduct alternately, the voltage operating range is narrow due to the energy balance. To expand the voltage operating range, an arm phase-shift modulation method is proposed in this paper. This paper presents an analysis of the operational process of T-AAMC to describe the relationship of the voltures and currents in DC and AC arms. Moreover, the power device, submodule capacitance and converter loss are discussed under the proposed topology and modulation. Finally, simulation results are provided to validate the effectiveness of the proposed topology and modulation strategy.