Constant PQ model or quasi-steady-state（QSS） model are widely used for HVDC links in the large-scale AC/DC power system low frequency oscillation（LFO） analysis. An important concern of the two types of HVDC models is that to what extent the calculation results are impacted by using the constant PQ model. The differences between the two models are investigated by a modal analysis method based on the augmented system state equation（ASSE）. The calculation results using the two models are compared in a LFO analysis. The theoretical and simulation results indicate that， when the HVDC system operates under the constant power control， the active power that the converters inject into or withdraw from the AC system is approximately constant， while the reactive power drawn from the AC system is varied. For the inter-area mode， the results from the constant PQ model are more conservative than that of the QSS model. For a large-scale AC/DC power system， it is acceptable to represent the HVDC links by the constant PQ model.