This paper investigates a control strategy that uses a multi-terminal high voltage direct current (HVDC) system for frequency control. It first describes a DC voltage control law which, based only on local information, makes the interconnected non-synchronous systems share their primary reserves. Then, a sufficient condition for choosing the controller gains is developed based on Lyapunov theory and LMI techniques with rigorous mathematical stability proofs. Simulation results show that the control strategy has achieved the objective of sharing primary reserves, guarantees the whole system's ultimate boundedness and improves the transient and steady-state performance.