Real-time assessment of the risk of continuous commutation failure of a thyristor-based Line-Commuted Converter High Voltage Direct Current System (LCC-HVDC) and timely preventive control measures are of great significance in ensuring safe and stable operation of a large power grid. This paper proposes a method for evaluating effective dynamic reactive power reserve of generators to deal with DC continuous commutation failure. The maximum reactive power increment of the generator corresponding to the secondary drop of the converter bus voltage is defined as the effective dynamic reactive power reserve, at the critical point this is called the maximum effective dynamic reactive power reserve. When the system dynamic reactive power reserve is less than the reserve capacity threshold, a two-stage preventive control measure search method is adopted. First, the method of increasing the generator terminal voltage and increasing the static reactive power compensation is adopted. If the requirements are still not met, the start-up units are increased. An example for an actual power grid verifies the effectiveness of the method. This work is supported by the Science and Technology Project of the Headquarters of the State Grid Corporation of China (No. 5100-201940013A-0-0-00).