Abstract: After a blocking fault occurs on one of DC transmission lines in a multi-DC delivery system, the stability of the system after disturbance can be improved by emergency DC power support (EDCPS) control on other non-faulted DCs. There is a lack of corresponding indexes on how to effectively quantify the improvement effect of EDCPS on system stability so as to improve the efficiency of safety and stability control; meanwhile, the existing studies have not yet solved the problem of how to maximize the integrated multi-DC sending capacity by using EDCPS. In this paper, the extended equal area criterion (EEAC) is applied to quantitatively analyze the mechanism of EDCPS to improve the transient stability of multiple DC transmission systems. The sensitivity of the EEAC-based transient stability margin to the EDCPS is studied in detail using a two-delivery DC system as an example. On this basis, this paper proposes a multi-DC sending capacity enhancement control method that utilizes the existing stabilization measures in the system and combines the transient stability margin sensitivity without increasing the related equipment inputs. By calculating the transient stability margin sensitivity of the alternative DC, the dynamic characteristics of the system after suffering from a large disturbance can be completely reflected, and the effect of the EDCPS on the improvement of the stability of the system after being disturbed can be quantitatively assessed, thus avoiding the problem of selecting the support DC only based on the operator's experience in the past and improving the calculation efficiency. Finally, the Northwest Power Grid is selected to simulate and verify the effectiveness of the proposed control method which enhances multi-DC integrated transmission capacity, to provide useful references for scholars and engineers in stability analysis and control of AC-DC hybrid power systems.