The parameter design and fault analysis of high voltage direct current (HVDC) system depend on modeling and simulation. The simulation of traditional detailed model usually needs real-time digital simulator and other equipments. However,it is difficult to ensure the accuracy of fault state in the existing simplified model. Therefore,it is necessary to propose a new modeling method to reduce the transient error of simplified model,and then reduce the simulation difficulty and design cost of HVDC system. In this paper,an ordinary differential equation (ODE) model of HVDC based on line-commuted converter (LCC-HVDC) is proposed,and the fault state of the switching function is modified. Firstly,based on the theory of switching function,the ODE model of LCC is proposed. Then,the influences of natural commutation point offset and commutation failure on the switching function are analyzed,and a switching function modulation strategy considering fault state is proposed. Finally,the simulation error and operation efficiency of ODE mode under various fault conditions are calculated with simulation examples. The simulation results show that the ODE model proposed in this paper accurately reflects the various fault states of the converter valve,with small error and high efficiency.