Abstract: With the increasing penetration of renewable sources in the power grid at the sending terminal of an ultra-high voltage direct current transmission system (UHVDC), the system inertia level is significantly reduced, and the frequency stability problem is becoming more and more prominent. By integrating the frequency response characteristics of the UHVDC system and the traditional synchronous power sources, this paper proposes a multi-timescale coordinated control strategy to actively support the frequency of the sending terminal system, achieving the reasonable utilization of the frequency modulation resources and improving the frequency stability of the system. Combining the maximum frequency modulation reserve capacity of the sending terminal system and the unbalanced power after a disturbance, the system frequency demand under different scenarios is analyzed, and a multi-timescale frequency modulation control strategy for the UHVDC system is proposed so that it has the capability of both inertial support and primary frequency regulation. With the maximum spare capacity of the primary frequency regulation of the synchronous machine, the dead zone value of the frequency regulation control strategy of the UHVDC system is designed, realizing the adaptive switching of the inertia support and the primary frequency regulation of the UHVDC system under different scenarios, which achieves the full utilization of the frequency regulation capacity of the synchronous machine while ensuring the frequency stability of the AC system at the sending terminal. On this basis, the dynamic response characteristics of the coordination control strategy under different scenarios are analyzed, further highlighting its superiority over the existing control strategies. Finally, a simulation model of the renewable sources via the UHVDC transmission system is built on the RTLAB real-time digital simulation platform, verifying the effectiveness of the proposed frequency-supported coordination control strategy.