Based on the transmission system interconnected by Chongqing-Hubei back to back voltage source converter based HVDC (VSC-HVDC) and Longquan-Zhengping line commutated converter based HVDC (LCC-HVDC), an additional frequency coordinated control strategy of VSC-HVDC and LCC-HVDC is proposed. The strategy can improve the frequency stability of VSC-HVDC transmission network by reasonable switching frequency-power additional frequency controllers of VSC-HVDC inverter station and LCC-HVDC rectifier station. And a frequency-power additional frequency controller is designed for VSC-HVDC inverter station to further improve the frequency transient characteristics of the system. Aiming at the problems of reactive power fluctuation and frequent switching filters in LCC-HVDC rectifier station during primary frequency regulation, an additional reactive power controller for VSC-HVDC inverter station is designed to reduce the number of switching filters of LCC-HVDC and improve the voltage stability of power network during frequency regulation. Finally, an equivalent simplified model of practical grid is built in MATLAB/Simulink, and the effectiveness and adaptability of the additional frequency coordinated control strategy are verified by simulation.