The default HVDC model used in software usually cannot describe some particular characteristics of an actual HVDC system in the electromechanical transient simulation of large-scale AC/DC power systems. According to the actual HVDC system， using the user-defined（UD） model circumstance provided by the power system analysis software package（PSASP） and adopting the injected power method， two HVDC models used for power flow and transient stability simulation are established. The transient stability UD model includes not only the most control function models of the actual system， but also the assistant function models such as tap changer control of transformers（TCC） and reactive power control（RPC）. In order to analyze the characteristics of the UD HVDC model in different working conditions， the EPRI-7 nodes system is quoted as a calculation example with the correctness of the UD model proposed proved by the results of the simulation.