For the AC system, the artificial node voltage is used to express the LTC branch power equations. For the DC system, an artificial node is introduced to locate between the ideal transformer and commuting impedance. Some variables such as artificial node voltage amplitude corresponding to the converter branch are included to express the converter equation. The coupling equations integrate the quadratic AC and DC equations from the different coordinates into a united quadratic AC-DC ORPF model. The predictor-corrector primal-dual interior point method (PCPDIPM) is used to implement the optimization. The Hessian matrices in this model are constant and require to be computed only once in the optimal process, which speed up the calculation greatly. The simulation results show that the proposed method achieves a good convergence property than the conventional one.