This paper proposes a novel primary frequency regulation (PFR) scheme for Multi infeed HVDC (MIDC) system. First, a dynamic model for frequency response (FR) in MIDC system is carried out to involve LCC-HVDCs in frequency regulation. Then, a disturbance observer (DO) is proposed to estimate both the power imbalance and the full order states of the system. By combining a Kalman filter (KF), this DO gets an accurate estimation quickly with local frequency measurement. On the basis of the estimated disturbance and state variables, feedforward control schemes can be concluded to fully utilize the fast adjustability of LCC-HVDCs. In this paper, an model predictive control (MPC) based scheme is designed to coordinate LCC-HVDCs under constraints of HVDC capacity and frequency boundaries. As a result, the optimal power-sharing property of LCC-HVDCs is achieved to enhance the PFR dynamic performance of the AC main system. Further, the stability of the proposed method is analyzed using Lyapunov method. Finally, numerical simulations are concluded to verify the effectiveness and the merits of the proposed method, by comparing it to typical droop controls.