With the development of energy internet, large-scale intermittent renewable power integration brings challenge to operation of power grid, which increases the pressure of thermal power peak shaving and requirements for the operation of coal-fired units are getting higher. In this paper, the health degree of the unit for measuring the operating status of coal-fired units is designed, and a prevention and control strategy based on generator side to prevent and block cascading failures is proposed. Firstly, the health degree for evaluating the operating status of coal-fired units is proposed, and a multi-objective planning model for coordinating system risk and dispatch cost based on this index is constructed. The Pareto solution of the health threshold under multi-objective model is calculated by NSGA-II algorithm to get the Pareto frontier and then evaluate the solution set by entropy weight decision method to determine the optimal threshold. Secondly, by comparing the health of the unit with the optimal health threshold, it is determined whether the coal-fired units in the prevention or blocking phase can meet the load transfer conditions. Finally, generators dispatching adjustment schemes under strategies of preventive control and interdict control are put forward respectively based on health degree. The IEEE39 system is used as an example to verify the simulation. The results show that the control strategy can effectively realize the safe and reliable dispatching of the grid and avoid the occurrence of cascading failures. This work is supported by Distinguished Youth Fund of Natural Science Foundation of Hunan Province (No. 2017JJ1011).