Actively commutated current-source-converter (ACC) can avoid commutation failure and has black start as well as AC/DC fault ride-through capability, which is suitable for constructing novel DC transmission system. However, there are commutation capacitances at the AC outlet of the ACC valve, and the coupled transient behavior of its voltage influenced by filter inductance and AC system feed-in energy after fault, as well as the changes in the circuit structure caused by the valve commutation influenced by the control system will affect the DC fault current characteristics, and the existing methodology lacks the ability to accurately analyze the DC faults of the ACC-HVDC in a quantitative way, which is not conducive to the design of the system planning and protection strategy. To address the problems above, this paper proposes an ACC-HVDC DC fault current analysis method based on a full-stage unified time-domain simplified model, which takes into account the state change of the LC filter under the action of the AC system as well as the bridge commutation, and is able to effectively improve the calculation accuracy. Meanwhile, the DC fault characteristics of ACC are studied. Firstly, the characteristics of each stage after ACC DC fault are qualitatively analyzed. Secondly, a unified time-domain simplified model of each stage of post-fault DC current is derived, its development mechanism is analyzed, and a PSCAD electromagnetic transient model is constructed to verify the accuracy of the proposed method by comparison. Finally, the effects of flat wave reactor and force-retardation-angle on ACC DC short-circuit faults and the AC bus transient overvoltage phenomenon after converter blocking are investigated.