The mechanism of UHVDC emergency DC power support（EDCPS） to enhance transient stability is analyzed based on the CCEBC/EEAC method. The impacts of ZIP load， IM load， load composition， and load location on the EDCPS performance following large disturbances are studied. It is found that following a large disturbance， IM load absorbs more active power than ZIP load and the influence of the sending-end IM load on the post-fault EDCPS power-angel stability is completely opposite to that of receiving-end load. The sending-end loads are helpful to enhance the system stability. In the case that the sending-end load is smaller than the receiving-end load， synthesis load tends to deteriorate the post-fault EDCPS effect， and the active power support needs to be increased or provided in good advance.